Fix loading MLV (Magic Lantern Raw Video).

Currently, FFmpeg (not libav) libswscale only supports de-Bayering to
8-bit rgb24 or yuv420p, and the colors are still wrong:
http://magiclantern.fm/forum/index.php?topic=11566
This will be more interesting when we can convert the raw to 16-bit
float RGB for Movit.