Swizzle

[mccreery]

• Switch channels like the GLSL swizzling
• RGB, Luminance, Average mode for "destination"
• Set or clear a channel ("0" or "1" source)

[mccreery]

Some notes:

• Should use matrix multiplication to avoid lots of if statements in the loop
• UI can be simplified by just having one source (R, G, B, A, Luminance, Average) and one or more grouped destinations (R, G, B, A, RGB)
• Consider renaming to "Project Channel" or similar