* commit 'b77fffa1':
  h264: make slice threading work with deblocking_filter=1

again label, and SLICE_SINGLETHREAD error handling are preserved as that
SLICE_SINGLETHREAD can be raised on a remaining case.

slice_context_count is also kept since it's still in use.
Merged-by: Clément Bœsch <clement@stupeflix.com>