ENH: Non-CG blend modes

[iccir]

The following blend mode don't have direct mappings to CGBlendModes:
Layer, Add, Subtract, Invert, Alpha, Erase

Add is close to kCGBlendModePlusLighter, but handles the alpha channel differently.
Invert is close to kCGBlendModeDifference, but draws using all black.

Erase and Alpha may have equivalent Porter-Duff modes.

See:
http://www.kaourantin.net/2005/09/some-word-on-blend-modes-in-flash.html