Blend
Color blending. Inspired from the implementation of Ghostscript
type mode =
| Normal
| Multiply
| Screen
| Overlay
| ColorDodge
| ColorBurn
| Darken
| Lighten
| Difference
| Exclusion
val blend : mode -> int -> int -> int
blend blendmode src dst computes blending of one color component.
blend blendmode src dst
val f : mode -> int -> Color.rgb -> Color.rgb -> Color.rgb
f blendmode srcalpha src dst 8bit depth .
f blendmode srcalpha src dst
bug: no destination alpha support.