Rewrite spill supression to accept a user setable threshold value

Also switch to smooth interpolation and adjust alpha too

Signed-off-by: cambell <cambell_prince@sil.org>
Alter MIXER BLEND command to take chroma threshold + spread rather than start stop

Signed-off-by: cambell <cambell_prince@sil.org>
Merge branch 'chroma' into 2.0.3-cp