Fix FFv1 output with FFmpeg

Close #8691

Of course, because we _really_ need that many RGB formats...
avcodec: Always output frames

With the introduction of CODEC_FLAG_OUTPUT_CORRUPT incomplete frames
are silently dropped. Enable it to maintain the previous behaviour.

Close #10140

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
Contrib: use a simpler way to discriminate between libav and FFmpeg
Merge remote-tracking branch: upstream/master