Add renderer state defines to shader preprocessor

[Chaosus]

Closes godotengine/godot-proposals#10764

implements 4 built-in defines to shader preprocessor:

• CURRENT_RENDERER is either 0, 1, 2 (depends on current renderer)
• RENDERER_COMPATIBILITY is 0
• RENDERER_MOBILE is 1
• RENDERER_FORWARD_PLUS is 2

Using it is simple:

[lostminds]

Could this perhaps be extended to include a couple of renderer feature support flags based on current renderer (see godotengine/godot-proposals#6207) and not just current renderer? For me instance uniform support FEATURE_INSTANCE_UNIFORMS would be the only example I'm interested in. But maybe there are other features that are only available in some renderers that could benefit from such feature flags as well?

The use case would be similar to the idea behind this PR as I understand it, as well as the logic and place to add it I think. But such feature flags would allow shaders to adapt and start using features as support is added in the engine without the developer needing to keep track of what is supported in each renderer/shader type and change their shader code.

[clayjohn]

Could this perhaps be extended to include a couple of renderer feature support flags based on current renderer (see godotengine/godot-proposals#6207) and not just current renderer? For me instance uniform support FEATURE_INSTANCE_UNIFORMS would be the only example I'm interested in. But maybe there are other features that are only available in some renderers that could benefit from such feature flags as well?

The use case would be similar to the idea behind this PR as I understand it, as well as the logic and place to add it I think. But such feature flags would allow shaders to adapt and start using features as support is added in the engine without the developer needing to keep track of what is supported in each renderer/shader type and change their shader code.

I think that's an interesting idea, but is way out of scope for this PR.

As for instance uniforms, they are fully supported in all rendering backends now. So there should be no need for a flag

[clayjohn]

Looks good, we should make a corresponding docs PR before merging.

[tetrapod00]

I created the docs PR: godotengine/godot-docs#10231.

[Repiteo]

Thanks!