Improve uniform distribution point radius scaling.

The "Uniform Spatial Distribution" glyph mode doesn't work well for large spatial domains because, for whatever reason, the function for calculating the glyph's nearest point radius does not scale linearly wrt domain size. Instead, it scales like L^0.25 where L is model size.

This is a problem at least for geodynamic models, because we often work in MKS units, but have model dimensions that are hundreds or thousands of kilometers.

I might be missing something about the original logic, because it looks too convoluted to be an accident. As best I can tell, the current functionality showed up in commit add4b082 but doesn't have much documentation, so I'm just guessing about the original intent.

There may be a better way to fix this, but this patch at least gets the scaling right, and works much better for large spatial domains than the original one did.