Add DummyShader handling to Dummy RenderingServer to ensure shader parameters are saved in headless export

[clayjohn]

Supersedes: #76121

Fixes #66842 (including the MRP in #76121 (comment))

This introduces the minimal set of logic needed by get_shader_parameter_list() which is used by materials to figure out what shader parameters need to be exported.

We can get away with a single shader compiler for all shader types since we don't care about the generated code, we just need the uniforms.

Also, we need to validate the existence of RendererSceneOcclusionCull::get_singleton() in RendererSceneCull::free() now since that gets called to free certain shaders.

[BastiaanOlij]

LGTM, interesting use case, I think we would have run into this in the XR team as we're working on using CI to build APKs for standalone VR devices.

[YuriSizov]

Thanks!

[nikitalita]

The dummy renderer sets low_end to true, which is checked by ShaderCompiler/ShaderLanguage when compiling the shader. Won't this fail to compile the shaders if it uses high-end-only built-in functions and such?

[nikitalita]

Yup:

SHADER ERROR: Built-in function "findLSB(int)" is only supported on high-end platforms.
          at: (null) (:26)
ERROR: Shader compilation failed.
   at: shader_set_code (servers/rendering/dummy/storage/material_storage.cpp:90)

And when it fails to compile, it obliterates the shader parameters, too:

[akien-mga]

Cherry-picked for 4.2.2.