Wrong terrain tiles

[jailln]

Context

Terrain tiles have been refactored in #2440. Since then, they have a point with altitude 0.
Also, it seems that the skirt is no longer working.

Steps to Reproduce

Open any globe example and toggle wireframe mode in the debug menu

Expected Behavior

Before the PR, without terrain:

and with terrain:

Actual Behavior

Currently on master, without terrain:

and with terrain:

The broken skirt:

[HoloTheDrunk]

I had painstakingly found and squashed what feels like this exact bug while working on the PR, but that change got crushed in the squash... Fixing this ASAP.