h264: use templates to avoid excessive inlining

Instead of inlining everything into ff_h264_hl_decode_mb(), use
explicit templating to create versions of the called functions
with constant parameters filled in.  This greatly speeds up
compilation of h264.c and reduces the code size without any
measurable impact on performance.

Compilation time for h264.c on an i7 goes from 30s to 5.5s.
Code size is reduced by 430kB.

Signed-off-by: Mans Rullgard <mans@mansr.com>