Cardboard demo has missing terrain tiles

[lilleyse]

This seems to have been introduced with log depth and specifically this commit: d681755

viewer.scene.logarithmicDepthBuffer = false; does not fix it, so it might be not be caused by log depth itself but possibly other changes surrounding it.

@bagnell could you check this out?

[mramato]

This is a dupe of #6612, there are some links to potential fixes there, so we should probably have a look.