Add first cut at a shader based chroma key facility 1. Added a new CHROMA <GREEN|BLUE> [alpha blend start, alpha blend stop] command. 2. Added a glsl shader and integrated into the existing pixel shader.

Signed-off-by: cambell <cambell_prince@sil.org>
Use glsl clamp function and remove the last explict conditionals from the chroma shader

Signed-off-by: cambell <cambell_prince@sil.org>
wip chroma
wip chroma shader, and command
wip chroma
wip spill suppression
chroma: blend in alpha rather than 1.0.
Merge branch 'chroma' into 2.0.3-cp