Patch makes it possible to compile blender with recent ffmpeg
and libav libraries, mainly by getting rid of deprecated API.
Original patch by Campbell Barton with own modifications to
support compilation with older ffmpeg versions.
This patch could break compatibility of FFV1 videos playing
back in older players, mainly because of alpha support changes.
Preserving compatibility with such players became a headache
and think it's high time to get rid of workarounds here.
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