Updates to control mesh configuration

[Carifio24]

This PR makes a few changes regarding the mesh structure of our scatter and glTF exports. The list of changes here is:

• Allow setting the maximum number of points (spheres) per mesh in glTF scatter exports. If the number of spheres for a given material is lower than the maximum value, all of the spheres for that material are put into one mesh. While we don't show this to the user, the points per mesh values are 1, 10, 100, 1000, 10000, 100000, 1000000, and "all points."
• Implement this mesh chunking for voxel exports as well. In my testing, exports using more voxels per mesh are often smaller, in addition to being more performant. Thus, I didn't add an option for the user to adjust this. I suspect that this is because there are generally so many voxels that removing the buffer views and accessors actually saves more space than it costs to add the additional triangle indices to the buffer.

Overall, the effect of this is to somewhat replicate the effects of Meshopt compression. While it makes sense for a user to use compression when they can, this is helpful for cases where the user wants to load the exported file into a client that doesn't support compression extensions.