When called on a link with a filter whose destination pad has not a
get_video_buffer callback defined, it will call
avfilter_get_video_buffer() on the first output link of the
destination filer, rather than use avfilter_default_get_buffer(), so
the video buffer can be allocated forward in the filterchain.

Also add the w and h parameters to avfilter_get_video_buffer(), as the
minimum width and height requested by each filter in the filterchain
may change, this allows for example a memcpy-less pad filter.

This change breaks API / ABI backward compatibility.

See the thread:
"[PATCH] Implement recusive avfilter_get_video_buffer()".

Originally committed as revision 20272 to svn://svn.ffmpeg.org/ffmpeg/trunk