Height query improvements

This also greatly improves lat/lon accuracy because it now finds the terrain position - not ellipsoid position - that the mouse cursor is over.

When moving the mouse, an approximate height is shown with an error bar.  The error can be pretty large when zoomed out:

![image](https://cloud.githubusercontent.com/assets/924374/3729679/a4821f88-16c1-11e4-89ea-162b811f8af1.png)

When you get in close, the error is much more reasonable:

![image](https://cloud.githubusercontent.com/assets/924374/3729685/b7f850aa-16c1-11e4-8f94-cf7faf959f10.png)

When the mouse stops moving for 250 milliseconds, we query the most detailed tile available for the position the mouse cursor is over, and display that height (without an error bar).

![image](https://cloud.githubusercontent.com/assets/924374/3729768/f5571bc4-16c2-11e4-8ccd-c6b0673f2383.png)

Heights are still referenced to WGS84, not mean sea level.  It will take me a little time to fix that because I'll need a mean sea level surface representation, like [EGM96](http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/egm96.html), on the client.

I think this is an improvement, but feedback welcome.  Lon/lat is hands-down better.  Height while moving the mouse is no longer misleading, at least, even if it's not especially useful when zoomed out.  Height after the mouse stops is the most accurate our terrain data lets us provide.  It should be a performance improvement as well.