Implement instance uniforms in Compatibility renderer

[clayjohn]

Cherrypicked from: #96814

The infrastructure for this has been in place for a long time, just the final details were needed to make it work.

• Production edit: This closes Add instance shader uniforms support/fallback in Compatibility renderer godot-proposals#6909.

[clayjohn]

@akien-mga This is a really trivial PR, we can probably go ahead with it without getting a rendering team approval.

[mhilbrunner]

✔️ Code looks fine
✔️ Style looks fine
✔️ Tested: Builds, runs, and feature works fine on Windows, behold:

With this, I can now switch between Compatibility and Forward+ and this scene works in both.

Docs are here: https://docs.godotengine.org/en/stable/tutorials/shaders/shader_reference/shading_language.html#per-instance-uniforms

[JekSun97]

Thanks!

[ClintochX]

Thanks so much.