H.264 decode: support cropping up to 28 pixels in interlaced mode.

H.264 decode: support cropping up to 28 pixels in interlaced mode.
Contrary to progressive, just being able to crop up to 14/15 pixels
is not enough to encode all supported resolutions, and the new
behaviour is also consistent with e.g. MPEG-2 etc.

Originally committed as revision 25669 to svn://svn.ffmpeg.org/ffmpeg/trunk