Defer linearization/dithering/bit-combining to second shader

[Jeremy Hill]

Deferred rendering: render current pipeline to a frame-buffer, then render the result as a single quad with a single texture and run it through the shader again to perform linearization/dithering.

Good for linearizing/dithering/bit-combining the result of stimuli that have been alpha-blended together.

Rendering to frame-buffer texture seems to work on macOS in --legacy mode despite nominally requiring OpenGL 3+. This is another thing (like glFenceSync) where we should test to see whether Linux is pickier than macOS, and also see how older versions of macOS behave.