Make avfilter_get_video_buffer() use

Make avfilter_get_video_buffer() use
avfilter_default_get_video_buffer() if the get_video_buffer() callback
is not defined in a filter.

Libavfilter filters author have to explicitely define the
get_video_buffer() callback if they want the buffer to be requested to
the filter following in the filterchain.

See the thread:
"[FFmpeg-devel] [PATCH] Make avfilter_get_video_buffer() use
default_get_video_buffer if callback not defined".

Originally committed as revision 20444 to svn://svn.ffmpeg.org/ffmpeg/trunk