Add viewport based scaling support

[temx]

When r_simplescale is set to 1, instead of adjusting texture LOD and point sampling in the screen effects shader, adjust the viewport to render a smaller rectangle and then point upscale after screen effects, before the HUD is drawn (like QuakeSpasm does). This gives faster 3D view rendering and supports any scale factor.

[Novum]

If we merge this then we should remove all the other scaling code. I was hesitant doing this before because it requires the GUI to be rendered in a separate render pass instead of a sub pass. I assume this is the case here?

[temx]

Unless I'm misunderstanding something, the GUI is already rendered on a separate render pass. The Screen Effects compute shader runs in the middle (and if using WATER_WARP it already reads from different coords), and that's where the scaling is done. The only use of subpasses that I can see is GUI -> POST_PROCESS (color correction), but that's after scaling is finished.

[Novum]

I'll take a look

[Novum]

Still want to merge this btw. Just haven't had time to look at it.

I basically want to verify first if it's not better to just realloc the render targets at the smaller size?

[temx]

IIRC using smaller render targets would require more of them (two are needed to run the screen effects, and if they're smaller a new larger one wold be needed for the UI), and managing that doesn't look like it'd be less complex. Is that just to save some memory?

[Novum]

I just have bad memories from using resolution scaling based on viewports. But I think we don't do any bilinear sampling of render targets (which causes problems at the borders) so it might be fine.
You can alias the same memory for multiple render target sizes. Just needs a barrier.

[Novum]

I'm okay with merging this, but I think we should just remove the screen effects path completely with the same PR.

[temx]

The subgroup stuff too? The screen effects path could be repurposed to do "supersampling downsampling" as it did before. Not sure if many people would want to use that.

[Novum]

I would just kill it. I'm not attached to that code :)