mp3dec: fallback to generic seeking when a TOC is not present

Fixes seeking without a Xing/Info header.

CC: libav-stable@libav.org
Signed-off-by: Anton Khirnov <anton@khirnov.net>
ff_get_buffer(): allocate the frame for max(coded,display) dimensions

Needed e.g. for h264 cropping to work properly.
h264: fully support cropping.

Based on a patch by Vittorio Giovara <vittorio.giovara@gmail.com>

Fixes Bug 378.