lavc/utils: add VP6F hack for setting correct video size in stream

For some obscure reasons avcodec_set_dimensions() resets the width/height
so that the resulting value set in the stream is equal to the
corresponding coded_width/height, which is not the same as the correct
width/height in case of the H.264 and VP6F codecs.

This adds a codec-specific hack for VP6F which disables the call to
avcodec_set_dimensions() in avcodec_open2(), like it is done with H264. A
proper fix needs to be found yet.

Fix trac ticket #1386.

Based on a patch by Michael Niedermayer.

Trolled-By: Daemon404

See thread:
Subject: [FFmpeg-devel] [PATCH] ffprobe: Stash and use width and height before opening the codec
Date: Fri,  1 Mar 2013 10:41:34 -0500
