Shaderblend code cleanup

[hrydgard]

Shader blend is really two separate concepts that have gotten intermingled in the code:

• Read from framebuffer in shader (copy or directly, depending on shader)
• Actually do the blend equation inside the shader (sometimes necessary for PSP-specific weirdness)

The second requires the first, but the first could also be used for other things (color bitmasks, soon), so separate the naming and clean things up a bit.

Also deletes reporting code for weird color masks. We know what these are and how they work..