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)

This index manipulation is a bit ugly.  If the existing looping setup has a measurable performance impact then you could enumerate from dirty.Count - 1 down to 0 to avoid this.  If the existing loop doesn't have a measurable performance impact then there is no real reason to change it.