Implement instance uniforms in Compatibility renderer

[clayjohn]

Cherrypicked from: https://github.com/godotengine/godot/pull/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 https://github.com/godotengine/godot-proposals/issues/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.

[TheOathMan]

anyone having issues getting instance working on Galaxy s25 ? They don't seem to work for me on S25 brand. So ultra and the none ultra as well. Instance works fine with Pixel and older Samsung devices.

Compatibility - Using Device: Qualcomm - Adreno (TM) 830