Add an option for selecting the core where the single thread of
checkasm runs. This allows benchmarking on specific CPU cores on
heterogenous CPUs, like ARM big.LITTLE configurations.

On Linux, one can easily wrap an invocation of checkasm with
"taskset -c <n> [...]" - so this option isn't very essential
there - however it is quite useful on Windows.

On Windows, it is somewhat possible to do the same by launching
the tool with "start /B /affinity <hexmask> [...]", but that
doesn't work well with scripting ("start" returns before the
command has finished running, and it's not obvious how to
invoke "start" from within WSL).

Using "taskset" to launch processes on specific cores within WSL
on Windows doesn't work - regardless of the Linux level affinity,
the process ends up running on the performance cores anyway.

The implementation is a hybrid between two approaches; one generic
(but non-ideal) for cases with large max_base_y, which fills two
pixel columns at a time, i.e. looping over pixels first vertically,
then horizontally - i.e. in a non-optimal manner.

For cases with smaller max_base_y, it does two rows at a time, essentially
doing gathers with the TBX instruction.

Relative speedup over the C code:

                         Cortex A53    A55    A72    A73    A76   Apple M1
intra_pred_z3_w4_8bpc_neon:    3.32   2.89   2.78   3.52   2.52   9.67
intra_pred_z3_w8_8bpc_neon:    6.24   5.55   4.76   5.60   4.11   6.40
intra_pred_z3_w16_8bpc_neon:   7.64   7.07   4.37   6.23   4.18   8.60
intra_pred_z3_w32_8bpc_neon:   7.51   7.21   4.34   5.92   4.27   7.88
intra_pred_z3_w64_8bpc_neon:   6.82   6.25   4.08   5.83   3.52   7.31

Relative speedup over the C code:

                         Cortex A53    A55    A72    A73    A76  Apple M1
intra_pred_z1_w4_8bpc_neon:    4.09   3.15   3.63   4.16   3.27  13.00
intra_pred_z1_w8_8bpc_neon:    6.93   5.66   5.57   6.76   5.51   5.50
intra_pred_z1_w16_8bpc_neon:   7.81   6.85   6.24   7.78   6.59   9.00
intra_pred_z1_w32_8bpc_neon:  10.56   9.95   8.72  10.95   8.28  13.33
intra_pred_z1_w64_8bpc_neon:  11.00  11.38   9.11  11.62   8.65  14.61

(The speedup numbers for M1 are kinda noisy due to the very coarse
granularity of the timer used there.)

These functions contain a number of different codepaths; try to
make sure that we hit most codepaths for each size combination.

This both gives better test coverage in one single run of checkasm,
but also should give a better averaged runtime in benchmarks.

Fixes #416.

The intent was good, but in practice it results in a significant
amount of problems due to various compiler bugs for negligible gains.

Should be useful for scenarios like wanting only keyframes to quickly generate
a set of preview images of the whole stream.

bits_left could underflow after reaching EOB.

Credit to OSS-Fuzz.

A length of 1 is by far the most common case, and having a special case
for that is not only slightly faster but also reduces code size by a
decent amount due to not having to pass a length argument every time.

The Dav1dSequenceHeader struct is already zero-initialized,
so zeroing individual values a second time is redundant.

According to section 6.4.1 of the AV1 specification, the value should
be equal to BUFFER_POOL_MAX_SIZE (10) when not explicitly signaled.

Fixes segfaults if you run the CLI with an invalid argument for --inloopfilters

Fixes: #412

It mirrors what is done with neon as well.

Fixes: #413

clang-15 doesn't consider it compile-time-constant anymore.

Already implied when -D_POSIX_C_SOURCE is not passed.

Non-GNU systems enable extensions (XSI, BSD, GNU) by default.

This fixes building with MSVC (and older GCC versions) after
3e7886db.

The completion of the first frame to decode while an async reset
request on that same frame is pending will render it stale. The
processing of such a stale request is likely to result in a hang.

One reason this happens is the skip condition at the beginning of
reset_task_cur().
=> Consume the async request before that check.

Another reason is several threads producing async reset requests in
parallel: an async request for the first frame could cascade through the
other threads (other frames) during completion of that frame, meaning
not being caught by the last synchronous reset_task_cur() after
signaling the main thread and before releasing the lock.
=> To solve this we need to add protections at the racy locations. That
means after we increase first, before returning from
reset_task_cur_async(), and after consuming the async request.

Despite not being documented in Meson's list of canonical system names,
Meson does accept 'ios' mostly a synonym for darwin.

By using 'ios' instead of darwin, it allows distinguishing between the
two in the cases where that is necessary. Therefore, within dav1d, allow
using the 'ios' name as alias for 'darwin' for system name, to allow
using cross files that does this distinction.

meson itself also allows 'tvos' in addition to 'ios' in the internal
`is_darwin()` function, as such all 3 are handled the same here.

'-fvisibility=hidden' only applies to definitions, not declarations,
so the compiler has to be conservative about how references to global
data symbols are performed.

Explicitly specifying the visibility allows for better code generation.

Whitespace is added to the result if compiling with MSVC using /std:c11
which breaks various things. Adding strip() fixes the problem.

Use explicit parameter type detection and manually clobber the
upper bits instead of relying on internal compiler behavior.

The 32-bit width parameter was used directly as a pointer offset, but
the upper half is undefined. Fix it by replacing 'cmp' with 'sub' to
explicitly zero those bits.

This fixes conformance with the argon test samples, in particular
with these samples:
    profile0_core/streams/test10100_579_8614.obu
    profile0_core/streams/test10218_6914.obu

This gives a pretty notable slowdown to these transforms - some
examples:

Before:                                 Cortex A53       A72       A73    Apple M1
inv_txfm_add_8x8_dct_dct_1_10bpc_neon:       365.7     290.2     299.8    0.3
inv_txfm_add_16x16_dct_dct_2_10bpc_neon:    1865.2    1384.1    1457.5    2.6
inv_txfm_add_64x64_dct_dct_4_10bpc_neon:   33976.3   26817.0   24864.2   40.4
After:
inv_txfm_add_8x8_dct_dct_1_10bpc_neon:       397.7     322.2     335.1    0.4
inv_txfm_add_16x16_dct_dct_2_10bpc_neon:    2121.9    1336.7    1664.6    2.6
inv_txfm_add_64x64_dct_dct_4_10bpc_neon:   38569.4   27622.6   28176.0   51.0

Thus, for the transforms alone, it makes them around 10-13% slower
(the Apple M1 measurements are too noisy to be conclusive here).

Measured on actual full decoding, it makes decoding of 10 bpc
Chimera around maybe 1% slower on an Apple M1 - close to measurement
noise anyway.

Using smaller immediates also results in a small code size reduction in
some cases, so apply those changes to the (10bpc-only) SSE code as well.

Certain clips were incorrectly performed on negated values, which
caused things to be off-by-one in both directions. Correct this by
negating such values prior to clipping instead of afterwards.

Since meson 0.58.0 (released in May 2021), meson accepts adding '.S'
assembly files as source files to the clang-cl compiler.

If using an older version of meson, keep using gas-preprocessor
just like for MSVC builds.