h264: don't clobber mmco opcode tables for non-first slice headers.

Clobbering these tables will temporarily clobber the template used
as a basis for other threads to start decoding from. If the other
decoding thread updates from the template right at that moment,
subsequent threads will get invalid (or, usually, none at all) mmco
tables. This leads to invalid reference lists and subsequent decode
failures.

Therefore, instead, decode the mmco tables only for the first slice in
a field or frame. For other slices, decode the bits and ensure they
are identical to the mmco tables in the first slice, but don't ever
clobber the context state. This prevents other threads from using a
clobbered/invalid template as starting point for decoding, and thus
fixes decoding in these cases.

This fixes occasional (~1%) failures of h264-conformance-mr1_bt_a with
frame-multithreading enabled.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>