huffyuvdec: implement trick for BGR

Before:
179436 decicycles in rgb, 32735 runs, 33 skips
Stripped object size: 23188

After:
154861 decicycles in rgb, 32738 runs, 30 skips
Stripped object size: 22736

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>