Use a list to track which fragments coded in this frame still have

Use a list to track which fragments coded in this frame still have
outstanding coefficients yet to be decoded from the bitstream. Once a
fragment reaches end-of-block, remove it from this new list. This change
makes the VP3/Theora entropy decode process dramatically faster due to
not having to iterate incessantly over fragments which have already been
fully decoded.

Originally committed as revision 20698 to svn://svn.ffmpeg.org/ffmpeg/trunk