Another requirement to have transparency in videos. Straight port of Firefox 53 implementation, save for some oddities relating to needing to put uint8_t instead of uint8 in yuv_convert to get this to compile.
Ref: BZ 1321076, 1329104
Encapsulate code from WebMDemuxer to query keyframe and frame resolution inside VPXDecoder, so we have a clean wrapper for all the libvpx functions we use.