Use the new idct functions (except chroma as it was slower in benchmarks)

Use the new idct functions (except chroma as it was slower in benchmarks)
cathedral +0.5% speed
aladin +0.6% speed [note aladin has been cat-ed 10 times to reduce the influence
of init time]
Speedup also verified via START/STOP_TIMER (difference was very significant
for the changed parts)

Originally committed as revision 16207 to svn://svn.ffmpeg.org/ffmpeg/trunk