Chroma Key

This patch adds a command to the mixer for chroma key a.k.a green (or blue) screen.  For example:

MIXER 1-10 CHROMA GREEN 0.10 0.04

where:
CHROMA is the new mixer command
GREEN is the color component to key on (GREEN | BLUE)
0.10 is the threshold center point
0.04 is the spread about the center point.