lavc: allow the caller to override dimensions in ff_get_buffer()

This will be useful for allocating edges in the encoders.
Merge commit 'f7e85ee996b3886c2b13e928b83277382311af96'

* commit 'f7e85ee996b3886c2b13e928b83277382311af96':
  lavc: allow the caller to override dimensions in ff_get_buffer()

Conflicts:
	libavcodec/utils.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpegvideo_enc: disable direct mode in load_input_picture() for dimensions%16 != 0

We currently read a whole 16x16 block from the input at a few places

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