Can be disabled by removing #define ALLOW_NOCHROMA in case the extra if()
slow the code down measurably.
Fixes at least
FRExt/HPCAMOLQ_BRCM_B.264
FRExt/HPCVMOLQ_BRCM_B.264

Originally committed as revision 14407 to svn://svn.ffmpeg.org/ffmpeg/trunk