Re-add terrain support.
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Fix Mercator projection in Columbus View and morphing.
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Fixes to work with latest code.
Fix rendering problem in some demos.

The Earth would be rendered in the wrong spot because a previous primitive
had set the model matrix to something other than identity.
Merge branch 'imagery_layers' into terrain
Add support for non-tiled ArcGIS MapServers.
Merge branch 'imagery_layers' into terrain
Merge branch 'imagery_layers' into terrain
Enable depth test when rendering terrain.