* commit '38efff92':
  FATE: add a test for H.264 with two fields per packet
  h264: fix decoding multiple fields per packet with slice threads

This merge includes two commits because the FATE test was useful in
order to make proper testing.

The merge gets rid of the now unused:
- SLICE_SINGLETHREAD and SLICE_SKIPED macros
- max_contexts
- "again" label in decode_nal_units()

This commit also includes the fix from d3e4d406.

Thanks to wm4 and Michael Niedermayer for their testing.
Merged-by: Clément Bœsch <u@pkh.me>
Merged-by: Matthieu Bouron <matthieu.bouron@gmail.com>