Dragging in Mercator projection results in unexpected movement

[zglueck]

Child of #509

Mouse drag input does not match globe movement when using the Mercator projection and the eye altitude less than 60km.

[pdavidc]

In light of Gerardus Mercator's birthday, I dug into this issue. This appears to be a symptom of a broader problem with our Mercator implementation. The cartesian to geographic conversion produces latitudes within 10m of the correct latitude.

Comparing our cartesian to geographic code with Snyder's reference shows that we've omitted a series in part of our computation. I recall Tom mentioning this, so it's possible that something about this series is problematic. It's also possible that he ran out of time to complete this work, and it's in an unfinished state.