Improved ore rendering

Originally I wanted to fix #6278 but I can’t reproduce the bug. I set all ore to the lowest density and mining it worked flawlessly.

But I fixed some other problems with ore drawing:
•	The sprite for the highest possible density was never used.
•	Added rounding to the ore density linear interpolation (old: 4.9 => 4.0 new: 4.9 => 5.0)

Do we need the additional `- 1`'s in the densities now that we are using the float version and rounding? `(float)t.Density / t.Type.Info.MaxDensity` would be a bit simpler.