Implement KHR_materials_dispersion

[Nemi26]

Hi, this commit implements the gltf dispersion extension. Partially fixes https://github.com/google/filament/issues/8307

Dragon Dispersion demonstration:

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[pixelflinger]

@Nemi26 also you need to add to the release notes

[pixelflinger]

oh, and also I think you need to add a unit test for that property.

[pixelflinger]

@Nemi26 see my comments above.

[Nemi26]

@pixelflinger I added the things you mentioned, as well as some tiny fixes (checking that refraction type is solid in order for dispersion to be enabled). I wasn't able to pull out that refraction inner loop code into a separate function in a way that would make it more readable unfortunately.

[pixelflinger]

oops, looks like all builds are broken.

[pixelflinger]

@Nemi26 the dispersion doesn't work with gltf_viewer's -u (ubershader) option; it crashes. FYI. Do you want to give it a shot at fixing it, or should I revert the change for now?

Also, I do see the effect working with khronos' DragonDispersion test, but not with the DispersionTest.