Camera goes under terrain

[rahwang]

Reported in the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/gqaHh1IaTQM

Looks like the camera goes under the terrain when the tiles are not fully loaded.

[hpinkos]

To reproduce, open the PA Terrain sandcastle demo, fly to one of the locations, then immediately middle mouse drag to tilt up. This moves the camera under the terrain.

[mramato]

This is by design, camera flights disable collision with the terrain until all of the terrain in the area has finished loading, otherwise it's impossible to have the camera fly anywhere close to the ground. See #4105