Make findHeightToViewBounds:pos work on MaplyViewController

I've got findHeightToViewBounds working, the problem was in checkCoverage. Even when pos was the center of the MBR the points that checkCoverage was checking were always off to one side, so the height was always to high, but not by a predictable amount.

This solution may not be what your looking for because it assumes the pos is the center of the MBR, but on the plus side it is faster because it makes fewer calls to screenPointFromGeo: