newer libav versions fail to decode vp9

[Antoine Martin]

Which is a big problem since that makes us fail the whole dec_avcodec2 module, which means no h264 decoding...
Note: vp9 + YUV444 was already broken before, which is why it is disabled in the code.

We need to:

• not fail the whole decoder, just remove the encodings that fail the self tests
• find out why it fails and fix it!
• re-enable YUV444 (lower priority)

[Antoine Martin]

• r9786 allows the dec_avcodec2 to continue decoding other video encodings and only disables the one that fails
• testing all the versions I have for Fedora 22:
  • 2.7.3 is ok
  • 2.6.3 is ok
  • 2.4.7 fails:

dec_avcodec.Decoder({...}).decompress_image(<type 'str'>:45, {}) avcodec_decode_video2 failed to decode the stream

• 2.6.1 is ok

So I am lowering the priority: we've been shipping ffmpeg>=2.6 for a while now.

[Antoine Martin]

Re-enabled VP9+YUV444P decoding in r9789 and added some warnings when older versions of ffmpeg are found:

• libavcodec version (56, 1, 100) is too old: disabling VP9 for versions older than 2.6.x (56, 26, 100)
• libavcodec version (56, 1, 100) is too old: disabling VP9 YUV444P support for versions older than 2.7.x (56, 41, 100)

This will do, users hitting the messages should know what to do.