Almost-cosmetics: split out coeff_count from all_fragments struct into

a separate array.
For the OGG version at http://www.bigbuckbunny.org/index.php/download/
this results in about 50% speedup of unpack_vlcs, resulting in ca. 20% faster
overall speed.

Originally committed as revision 13729 to svn://svn.ffmpeg.org/ffmpeg/trunk