Small refactoring: Instead of 4 loops for decoding AC coefficients based

Small refactoring: Instead of 4 loops for decoding AC coefficients based
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().

Originally committed as revision 20699 to svn://svn.ffmpeg.org/ffmpeg/trunk