hevc_parser: Support bitstream encapsulated as 14496-15 format.

Note that the split function only supports Annex B format since we can't
pass the private data to the function.
hevc_parser: Remove duplicated key_frame setting.

key_frame setting in parse_nal_units() is better since it is not invoked
when PARSER_FLAG_COMPLETE_FRAMES is set.
hevc: frame mt

Conflicts:
	libavcodec/hevc.c
	libavcodec/hevc.h