Shader pre-processor: Improve parsing + fix WGSL

[Popov72]

I have improved the shader pre-processor so that constructions of this type are analyzed correctly:

#define BRDF_DIFFUSE_ROUGHNESS_EON 0
#define BRDF_DIFFUSE_ROUGHNESS_BURLEY 1

#if BASE_DIFFUSE_ROUGHNESS_MODEL == BRDF_DIFFUSE_ROUGHNESS_EON
    diffuseRoughnessTerm = diffuseBRDF_EON(vec3(1.0), diffuseRoughness, NoL, NoV, LoV) * PI;
#elif BASE_DIFFUSE_ROUGHNESS_MODEL == BRDF_DIFFUSE_ROUGHNESS_BURLEY
    diffuseRoughnessTerm = vec3(diffuseBRDF_Burley(NoL, NoV, VoH, diffuseRoughness) * PI);
#endif

Before the PR, you couldn't test against a name, you had to use immediate values (0 instead of BRDF_DIFFUSE_ROUGHNESS_EON and 1 instead of BRDF_DIFFUSE_ROUGHNESS_BURLEY in the example above).

I have also reworked the way we handle preprocessing in WGSL, as it was causing some problems before. I have tried to add a number of comments in the code to explain why things are done in that way.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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[bjsplat]

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[bjsplat]

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