hwaccel: consistent name prefixes for start_frame/end_frame/decode_slice

Some hwaccels use name prefixes, some do not, others only use them for
some codecs.  Add prefixes everywhere for consistency.
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  hwaccel: consistent name prefixes for start_frame/end_frame/decode_slice

Conflicts:
	libavcodec/vda_h264.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
libopusenc: use ff_alloc_packet2

Signed-off-by: Paul B Mahol <onemda@gmail.com>
libilbc: use ff_alloc_packet2

Signed-off-by: Paul B Mahol <onemda@gmail.com>
vc1dec: use av_log_ask_for_sample for odd sprites

Suggested-by: Reimar
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>