To support future MPEGs of uncertain layout and frame variations (namely
EALayer3) the code has been restructured: mpeg_decoder does stream
procesing and decoding, while mpeg_custom_utils_x does init and parsing
(write to data buffer + update offsets), per MPEG subtype. Internals
have changed but still gives byte-exact results.
AHX has been adapted to this format as a test. Some modes
(P3D/EALayer3/LyN/AWC) are defined for later development but will fail
on init ATM.
