Separate "layers" for shader API

[ThisGuyCodes]

Currently in custom shaders you get a iChannel0 uniform with the current fully-rendered terminal screen.

I noticed in poking around some example shaders that being able to distinguish the background is useful.

I was wondering if it would be feasible to expose more uniforms than just the fully-rendered terminal screen? E.g. default-background (where the color hasn't been overridden), background (where it has been overridden), foreground (i.e. the text)...

Some amount of this could be accomplished by setting a specific background color that the shader expects, but then you end up with a "can't show green with a green-screen" situation.