mp_msg: fix typo in message level for new msg API macros
video: add vda decode support and direct output

Decoding H264 using Video Decode Acceleration used the custom 'vda_h264_dec'
decoder in FFmpeg.

The Good: This new implementation has some advantages over the previous one:

 - It works with Libav: vda_h264_dec never got into Libav since th patch was
   submitted only to ffmpeg and it was anyway against Libav design direction
   which is for client code to use the 'cleaner' hwaccel API.

 - It is way more efficient: on my tests this implementation yields a
   reduction of CPU usage of roughly ~50% compared to using `vda_h264_dec` and
   ~65-75% compared to h264 software decoding. This is mainly because
   `vo_corevideo` was adapted to perform direct rendering of the
   `CVPixelBufferRefs` created by the Video Decode Acceleration API Framework.

The Bad: Now `vo_corevideo` is required to use VDA decoding acceleration.

The Ugly: VDA was hardcoded to use UYVY (2vuy) for the uploaded video texture.
One one end this makes the code simple since Apple's OpenGL implementation
actually supports this out of the box. It would be nice to support other
output image formats and choose the best format depending on the input, or at
least making it configurable. My tests indicates that CPU actually goes up
with a 420p output IMGFMT, even though I have really no idea why.

asd