Fix skip-lods for single-sided walls

Compare skip-lods off vs. on in [3D Tiles Interior](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/?src=3D%20Tiles%20Interior.html&label=Showcases)

This is an expected bug with the skip-lods algorithm. When the tileset has mixed refinement the back faces of parent tiles are rendered to lay down depth, but the back faces are culled in the color pass. The second screenshot is the result of that.

Reproduce the difference between skip-lods off and on using the linked 3D Tiles Interior Sandcastle example. Read the skip-lods algorithm and its handling of parent-tile back faces in the depth and color passes; done means single-sided walls render correctly with skip-lods enabled, matching the expected result.

Written by the indexing model from the issue text.