Add multiple renderer support to Advanced Postprocessing

[tetrapod00]

Addresses #10204.

Adds code for the Compatibility renderer to the Advanced Postprocessing page.

With godotengine/godot#98549 merged, there is now a way to detect the renderer in shaders that has low no runtime cost. Unlike using OUTPUT_IS_SRGB, it also is obvious what it does. Alternately, CLIP_SPACE_FAR could be used in the example here, since NDC depends directly on that value. But the math to do so is non-obvious, and using a renderer define here is more obvious and instructive.

[Calinou]

Looks good to me.

Note: Not cherry-pickable to 4.3, as this feature is only in 4.4.

[tetrapod00]

I can make a separate cherrypickable version that either

• does not mention supporting multiple renderers, only adds the Compatibility NDC snippet
• uses the jankier OUTPUT_IS_SRGB and runtime if() to detect the renderer

[mhilbrunner]

Thank you!