avcodec/h264dec: be more explicit in handling container cropping

avcodec/h264dec: be more explicit in handling container cropping

This merges commit 4fded0480f20f4d7ca5e776a85574de34dfead14 from libav,
originally written by Anton Khirnov and skipped in
fc63d5ceb357c4b760cb02772de0b50d0557140f.

 libavcodec/h264_slice.c | 20 +++++++++++++-------
 libavcodec/h264dec.c    |  3 +++
 libavcodec/h264dec.h    |  5 +++++
 3 files changed, 21 insertions(+), 7 deletions(-)