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".

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20444 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b