CLAMP_TO_GROUND for Billboards incorrect positions

When using a terrain provider that uses a heightmap terrain format, the CLAMP_TO_GROUND property produces incorrect positions for billboards. It is easier to see when there are more billboards placed. I can get it to happen every time when there are 10 billboards on the globe.

I've tested with the terrain provider at https://cesiumjs.org/smallTerrain. This is not an issue when the terrain provider provides a quantized mesh format.

I'm using the current 3d-tiles branch at revision 31cf664a04881bc618f468043370f8fcd254859b.

Start by reproducing the issue with the smallTerrain provider and about 10 CLAMP_TO_GROUND billboards. Compare billboard positioning with heightmap terrain against quantized mesh terrain, then trace the billboard ground-clamping path. Done means heightmap terrain produces correct positions without regressing the quantized mesh case.

Written by the indexing model from the issue text.