Basically, we need to make sure to allocate enough data for the pretty
dumb copy_nv12 function. (It could be avoided by making the function
less dumb, but this fix is simpler.)
This is basically a hack for drivers which prevent the mpv DXVA2 decoder
glue from working if OpenGL is in fullscreen mode.
Since it doesn't add any "hard" new API to the client API, some of the
code would be required for a true zero-copy hw decoding pipeline, and
sine it isn't too much code after all, this is probably acceptable.
This is pretty much copy&pasted from Libav commit
a7e0380497306d9723dec8440a4c52e8bf0263cf.
Note that if FFmpeg was not compiled with HEVC DXVA2 support or your
video drivers do not support HEVC, the player will not fallback and
just fail decoding any video. This is because libavcodec appears not
to return an error in this case. The situation is made worse by the
fact that MSYS2 is on an ancient MinGW-w64 release, which does not
have the required headers for HEVC DXVA2 support.
At least on my machine, reading back the frame with system memcpy is
slower than just using software rendering. Use the optimized gpu_memcpy
from LAV to speed things up.
