Added MMX optimized yuv2rgb (AlienSong now plays back at only 6% CPU)

Added MMX optimized yuv2rgb (AlienSong now plays back at only 6% CPU)

Original commit message from CVS:
Added MMX optimized yuv2rgb (AlienSong now plays back at only 6% CPU)
Added mpeg1 picture skipping and fixed a buffer overflow.
Added a system clock. The audiosink can now adjust the clock.
Fixed incorrect behaviour on 8, 15, 16, 24 and 32 bits displays.
Cleanup of the videosink, it now uses the color conversion library when
needed.