Known FFmpeg issues that probably should be reported/fixed upstream

[myrsloik]

1. Some FFmpeg encoders don't flag keyframes. The HuffYUV encoder is the most notable case of this. Every frame is a keyframe so why can't it be marked as such?
2. VC-1 decoder output isn't bitexact after seeking.
3. Somewhat corrupt MPEG2 files such as the one in Decoding artifacts,.. #31 don't produce bitexact output with repeated linear decoding. This probably indicates harmless uninitialized memory somewhere.