Previously, the streams' offsets needed to be pre-adjusted with the
interleave; now assumes all streams start in the same offset (first
stream).
This simplifies short last interleaves (SCD/P3D) and makes
layout_mpeg_custom unnecessary (also allows theoretical variable-sized
interleaves).
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.
