Disable the depth test when clamped to ground zooms to wrong location

Select `Disable the depth test when clamped to ground` in http://localhost:8080/Apps/Sandcastle/index.html?src=Billboards.html&label=All and you'll be looking at the size of a mountain. You need to move the camera to see the billboard.

This is using the `viewer.zoomTo`, so maybe we introduced a bug when clamping to ground?

Reproduce the issue in Sandcastle using Apps/Sandcastle/index.html with the Billboards.html example and the “Disable the depth test when clamped to ground” option. Start by tracing the viewer.zoomTo camera movement and clamped-to-ground behavior; done means the camera opens at the billboard instead of requiring manual movement.

Written by the indexing model from the issue text.