Define a new struct AVFilterBufferRefVideoProps and add a type field
to AVFilterBufferRef.

Video specific properties in AVFilterBufferRefVideoProps are now
referred to by *video pointer in AVFilterBufferRef.

Patch by S.N. Hemanth Meenakshisundaram smeenaks->ucsd.edu.

Originally committed as revision 24763 to svn://svn.ffmpeg.org/ffmpeg/trunk