Add ghost particle advection.

[entropylost]

This PR does a couple of things:

• Allows displaying the phase in the rendering.
  • In order to do this, I needed to make the particle phase struct available outside of the particle_update.wgsl file, so I moved it to both particle2d.wgsl and particle3d.wgsl; I don't think there's really much of a better place to put that.
• Adds ghost particle advection.
  • Advection does not handle rigid colliders properly right now.
• Made most of the storage tensors COPY_SRC. That's necessary for interfacing with other things and I don't see much of a reason for them not to be.
• Made an init_testebed_with_settings function that can take in some parameters.