This reverts commit b6104e12.

There's no real reason this needs to be used. Users would do just fine
without it, if they don't care about using the renderer and just want to
use this function as a convenient upload helper.

Since we no longer need the GCC pragma for symbol visibility, there's
also no more reason to have common.h import everything under the sun -
so only have modules import what they actually use.

This speeds up compilation a tiny bit.

This is much more convenient when the plane is different every time, but
you still want to re-use the same texture.

Exactly what it says on the tin.

No need to duplicate logic. I forgot this helper existed already.

This is a very major rewrite operation, but all of the actual logic is
unaffected. The change is completely cosmetic.

The idea behind this is to avoid clashing the mpv ra_ namespace when
libplacebo eventually makes its way back into mpv, allowing it to
coexist with vo_gpu peacefully (at least for the transition period).

It's also sort of weirdly inconsistent with the rest of libplacebo
anyway.

ffsll() is available in glibc and BSD, but it's non portable and
unavailable on Windows. Use __builtin_ffsll() instead. This function
already uses GCC builtins anyway.

This family of helpers will hopefully make it much easier for users to
pick suitable image formats and upload arbitrary data to the GPU.

In future, I want it to also support on-CPU conversions where necessary
(using e.g. zimg), but this is not implemented yet.