More reliably stretch the base layer over the poles.

From the terrain branch...

While working on the Santa Tracker, we noticed that the web mercator base imagery layer was sometimes not stretched over the geometry at the poles.  This change handles that by applying base layer textures to tiles that don't actually intersect the layer at all so that the final row of texels is stretched all the way over the pole no matter how close you zoom.  Prior to this change, zooming in close to the pole would show blue tiles.

There's little chance I'm going to write a test for this, sorry.