CABAC trellis opts part 2: C optimizations

CABAC trellis opts part 2: C optimizations

Hoist the branch on coef value out of the loop over node contexts.
Special cases for each possible coef value (0,1,n).
Special case for dc-only blocks.
Template the main loop for two common subsets of nodes, to avoid a bunch of branches about which nodes are live.
Use the nonupdating version of cabac_size_decision in more cases, and omit those bins from the node struct.
CABAC offsets are now compile-time constants.
Change TRELLIS_SCORE_MAX from a specific constant to anything negative, which is cheaper to test.
Remove dct_weight2_zigzag[], since trellis has to lookup zigzag[] anyway.

60% faster on x86_64.
25k->18k codesize.