Expose SplatMaterial.getVertexShaderFadeIn or similar

[hybridherbst]

For custom effects, it would be great to have a callback into shader generation. This can currently be achieved with some hackery (e.g. attach to three's shader compiler), but I think having a dedicated callback would be cleaner.

Even just exposing some way to monkey-patch into a shader method would be useful.

This could look similar to https://lumalabs.ai/luma-web-library (see "Custom Shaders", "Shader Hook API")

[mkkellogg]

I think this sort of capability would be great to have, it would make adding custom effects much easier and it could make it easier to solve issues like this one without creating a new release. Hopefully I'll have some time soon to add something like this!