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Set to NULL in y4m2.c:ss_names, as it is set
in chr_names_8bpc_i420.

fix #20

Fixes #21.

Open files in binary mode, or on Windows fread/fwrite will treat the files as text and mangle the read/written data

The build system declared `rev_target` to be a dependency of
libdav1d instead of libdav1dentrypoint. This causes initial builds
to fail, as it is `src/lib.c` from libdav1dentrypoint that includes
this file. Move the dependency to libdav1dentrypoint to fix this.

This is a fix for

    ERROR:  Unknown variable "stackrealign_flag"

when configuring for x86 on compilers that do not support any of the
checked stackalign flags.

Since some code is still taken from there

Always use the compatibility layer.
Fix #28

Since we don't want to probe for winpthreads anymore, this macro isn't
usefull anymore

Also a tiny fix to ensure that frame-difference is always evaluated
in the correct wraparound order.

Downshift all the constants by one, and reduce the rounding shift by one.
This is mathematically equivalent since all constants are a multiple of two,
but allows for using 16-bit intermediates in the 1st pass of the 8-tap filter.

This library is just a helper and should not be built by default as it
is only used to extract the objects and has no use as stand-alone
library for uses to link to.

This makes the function thread safe, to call, for example,
from multiple transient dependecies or threads.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

This function will be required to make dav1d_init threadsafe.

Full erro checking support is included in this implementation,
for completeness, even though dav1d_init cannot fail, and thus
will ignore the return value of pthread_once.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>