[Merged by Bors] - Fix dependency of shadow mapping on the optional PrepassPlugin

[geieredgar]

Objective

Unfortunately, there are three issues with my changes introduced by #7784.

1. The changes left some dead code. This is already taken care of here: [Merged by Bors] - Remove dead code after #7784 #7875.
2. Disabling prepass causes failures because the shadow mapping relies on the PrepassPlugin now.
3. Custom materials use the prepass.wgsl shader, but this does not always define a fragment entry point.

This PR fixes 2. and 3. and resolves #7879.

Solution

• Add a regression test with disabled prepass.
• Split PrepassPlugin into two plugins:
  • PrepassPipelinePlugin contains the part that is required for the shadow mapping to work and is unconditionally added.
  • PrepassPlugin now only adds the systems and resources required for the "real" prepasses.
• Add a noop fragment entry point to prepass.wgsl, used if NORMAL_PASS is not defined.

[geieredgar]

While investigating #7879, I have found a third issue: When using custom materials, the default prepass.wgsl shader could be used. But prepass.wgsl only defined an entry point for the fragment shader if NORMAL_PASS was defined. I added an entrypoint for the other cases that simply does nothing. This will produce the same shadows like before #7784. So a custom material should override the prepass_fragment_shader method to be able to obtain better shadows like the StandardMaterial.

[robtfm]

This does work, but I don't think adding the dummy fragment shader should be necessary. instead we can just not add the FragmentState in PrepassPipeline::specialize() when the material_fragment_shader is None.

StandardMaterial returns a handle, so that will behave correctly. Custom Materials that return None would not add a frag shader, which would be more efficient.

0.10 is looming though, so if there isn't time then this is fine as is

[geieredgar]

Ok, I adjusted the fragment state condition and its comment, reverted the fragment entry point commit and tested that the shader_prepass example still works. I think this PR is ready to be merged.

[alice-i-cecile]

bors r+

[bors]

Pull request successfully merged into main.

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