Load higher-level tiles before lower-level ones.

This is probably still not optimal, but it's much better and doesn't look nearly as obviously busted as it did before.  To see what I mean, run the "Imagery Layers Manipulation" Sandcastle example, turn off all layers except the base one, and zoom in to a horizon view.  Then turn on the infrared layer.  In master, you can watch the tiles load from far-to-near.  In this branch, they load near-to-far.  Much better.  Plus I was able to delete a bunch of code - hooray!