Record B-fraction index from frame header and two fields from entry point structure

into VC-1 context.

Patch by Gwenole Beauchesne
(gbeauchesne <whirlpool> splitted <dash> desktop <dit> com)
Thread [PATCH][VC-1] Record extra syntax elements for VA API support

Originally committed as revision 16859 to svn://svn.ffmpeg.org/ffmpeg/trunk