The HEVC decoder has both HEVCContext and HEVCLocalContext
structures. The latter is supposed to be the structure
containing the per-slicethread state.

Yet that is not how it is handled in practice: Each HEVCLocalContext
has a unique HEVCContext allocated for it and each of these
coincides except in exactly one field: The corresponding
HEVCLocalContext. This makes it possible to pass the HEVCContext
everywhere where logically a HEVCLocalContext should be used.

This commit stops doing this for lavc/hevcpred as well as
the corresponding mips code; the latter is untested.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>