Give arbitrary inputs to fragment shaders

[argv]

Currently, we have the ability to apply fragment shaders to arbitrary textures. This is quite useful! However, there's no way to give them additional uniforms from ZScript.

This makes it especially hard (and slow) to do complex effects on the status bar/HUD. I recently posted a request for help with drawing a stylized bar on the HUD, and while I was given a clever solution, it's still much slower than drawing the entire thing with a shader and a single draw call.

Please consider adding a drawing API to ZScript that applies a fragment shader to a region of the screen, and allows ZScript code to pass arbitrary uniform variables (preferably including additional textures) to the shader.

[Rachael]

Since Vulkan is done, the hwrenderer is no longer in a state of feature freeze, as far as I know. This has come up again, as a desired feature, maybe the idea can be revisited?