on their grouping, create one loop that indexes into a table of AC VLC 
tables.

There is also a small optimization here: Do not call unpack_vlcs()
if there are no fragments in the list with outstanding coefficients. 
My profiling indicates that this can save upwards of 1 million 
dezicycles per frame throughout the course of unpack_dct_coeffs().


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20699 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b