Texture swizzling

[ColonelThirtyTwo]

Luminance should support setting the TEXTURE_SWIZZLE_R/G/B/A texture parameters, defined in section 3.8.8 of the OpenGL 3.3 spec.

Should be fairly straightforward to implement in the shader code. Would make R8 textures easier to use as grayscale textures.

[hadronized]

The interface should be pretty simple to implement and the backends fairly easy to update, yes.