avcodec/smacker: Disentangle two contexts

avcodec/smacker: Disentangle two contexts

Smacker uses two types of Huffman trees: Those for eight bit values and
those for 16 bit values. Given that both return their values via arrays
and that both need to check not to overrun their array, the context for
parsing eight bit values (HuffContext) will necessarily exhibit certain
similarities with the context used for parsing 16 bit values (DBCtx).
These similarities led to using a HuffContext in addition a DBCtx for
parsing 16 bit trees.

This stands in the way of further developments for the HuffContext struct
(when parsing eight bit trees, the length of the arrays are always 256,
so that one can inline said value and move the currently heap-allocated
tables directly in the structure).

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>