avfilter_(un)ref_pic       -> avfilter_(un)ref_buffer
avfilter_copy_picref_props -> avfilter_copy_buffer_ref_props
AVFilterBufferRef.pic      -> AVFilterBufferRef.buffer

They have been renamed to allow sharing with audio.

Patch by S.N. Hemanth Meenakshisundaram $smeenaks$ucsd$edu$.

Originally committed as revision 24731 to svn://svn.ffmpeg.org/ffmpeg/trunk