feat: Allow re-use of existing materials

[hmans]

As of now, the materials for particles are always created from scratch (using <ParticlesMaterial>, which uses three-custom-shader-material to patch our shader code into newly created instances of Three's built-in materials.)

Ideally, we'd be able to patch existing materials. This is important for eg. using materials imported from a GLTF file.

This probably requires a PR to three-custom-shader-material in order for it to accept existing instances of materials.

[hmans]

An upcoming release of three-custom-shader-material will allow us to reuse existing materials, which pretty much auto-fixes this issue.

[hmans]

This now works, thanks to three-CSM 3.4.0.