Avoids allocations and error checks for these allocations;
e.g. syncing buffers across threads can't fail any more
and needn't be checked. It also avoids having to keep
H264ParamSets.pps and H264ParamSets.pps_ref and PPS.sps
and PPS.sps_ref in sync and gets rid of casts and indirections.

(The removal of these checks and the syncing code even more
than offset the additional code for RefStruct.)
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>