Use dithering when performing SSR roughness to improve quality

[Calinou]

• Follow-up to Jitter shadow map dithering pattern across frames when TAA is enabled #97428.

This works best when TAA or FSR2 is enabled to jitter the result every frame, letting TAA accumulation do its work.

Testing project: https://github.com/Calinou/godot-ssr-test

Preview

TAA is enabled on all screenshots. Images should be compared by opening them in a new tab, so that they can be viewed at 1:1 size.

Before

Low quality	Medium quality	High quality

After

Low and Medium quality settings now resemble High quality more closely, with less visible blockiness.

Low quality	Medium quality	High quality

TODO

• Use the TAA frame count from camera data, so that the jitter is disabled when TAA is disabled and the TAA frame count varies according to the Halton sequence length.
  • I'm not sure how to pass this from RendererSceneCull to SSEffects.
• Improve appearance when using FSR2, which has a harder time smoothing out dithering patterns that change over time.
  • See Jitter shadow map dithering pattern across frames when TAA is enabled #61834 (comment). I don't know how to apply this to SSR jittering, since it's slightly different in concept (no disk rotation is involved).