With the introduction of HEVC 444 support, we technically have two
codecs that can handle 444 - HEVC and MJPEG. In the case of MJPEG,
it can decode, but can only output one of the semi-planar formats.
That means we need additional logic to decide whether to use a
444 output format or not.
The 'simple' hwaccels (not h.264 and hevc) all use the same bitstream
management and reference lookup logic so let's refactor all that into
common functions.
I verified that casting a signed int -1 to unsigned char produces 255
according to the C language specification.
