The 3-bit field at +24bit in DWORD 1 of the XMA context was named
subframe_skip_count but never actually used for skipping subframes.
Testing across multiple titles (PGR4, Halo Reach) shows the field
might control extra output buffer blocks that must be reserved per decoded
frame. Renaming the field to output_buffer_padding to reflect this
observed behavior and adding padding to the minimum output space threshold
Pdding blocks are reserved from the output budget after each frame is
fully consumed, preventing the decoder from overrunning the space
that the game expects to remain free.
This is still likely not entirely correct but reduces some of the
observed noise in current implementation.
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