Implement forEachSplat and getBoundingBox for PagedSplats

[mrxz]

This PR moves the forEachSplat method into the SplatSource interface, ensuring it works across PackedSplats, ExtSplats and PagedSplats. This allows the SplatMesh.forEachSplat and SplatMesh.getBoundingBox methods to work for paged splats (see #317).

These implementations are somewhat unintuitive for paged splats, as they depend on which splat pages happen to be loaded. This might confuse users, though arguably the current behaviour of no-op for forEachSplats isn't intuitive either.

[asundqui]

That's great! Let's merge it I say! Surely someone will want to do this :) @dmarcos

[dmarcos]

Thanks