Implement global and per instance shader uniforms.

[reduz]

Adds two keywords to shader language for uniforms:
-'global'
-'instance'

This allows them to reference values outside the material.

Bugsquad edit: closes godotengine/godot-proposals#257, closes #18848, closes #30375

[Chaosus]

Some Travis error messages about fallthrough should be fixed (its easy), otherwise looks ok...

[Schroedi]

Per instance uniforms sound like instance parameters for MultiMeshInstances: https://docs.godotengine.org/en/3.2/tutorials/3d/using_multi_mesh_instance.html

I guess this is not what they are meant for, or can they be used there as well?

[reduz]

Oh just as a side note, per instance uniforms are not working for 2D (CanvasItems) because I could not find a way to implement them. I am planning to eventually rewrite the 2D rendering code to make it more efficient before 4.0 is out, now that I am more proficient with Vulkan so I will properly implement those there.

[akien-mga]

Thanks!