MB_TYPE_ACPRED is currently reused for MB_TYPE_REF0 by H.264,
so that the value fits into an uint16_t. Given that MB_TYPE_ACPRED
is not subject to any such restriction (apart from fitting into
32bits), it can be remapped to a hithereto unused bit.
The then available bit will be declared to be codec-specific
(i.e. unused by generic code), so that H.264 can use it
for MB_TYPE_REF0 and so that it can be reused later for
e.g. MB_TYPE_H261_FIL.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>