A work around ffmpeg issue that everyone (e.g. OBS) seems to be doing.
By default ffmpeg uses built-in VP8/VP9 decoders, however those
do not detect alpha channel (https://trac.ffmpeg.org/ticket/8344 -
the bug filed in 2019, currently still open in ffmpeg 7.1. There's
an older report from 2016 too, https://trac.ffmpeg.org/ticket/5792).
The trick for VP8/VP9 is to explicitly force use of libvpx decoder.
Only do this where alpha_mode=1 metadata is set. Note that in order
to work, the previously initialized format context must be closed
and a fresh one with explicitly requested codec must be created.
Pull Request: https://projects.blender.org/blender/blender/pulls/132795
29e8c0ac01