Rename fields:

AVFilterLink.srcpic    ->  AVFilterLink.src_buf
AVFilterLink.cur_pic   ->  AVFilterLink.cur_buf
AVFilterLink.outpic    ->  AVFilterLink.out_buf

The new names are more generic and more consistent, since the struct
they contain, which was named AVFilterPicRef, has been renamed to
AVFilterBufferRef.

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

Originally committed as revision 24732 to svn://svn.ffmpeg.org/ffmpeg/trunk