Handle `skipLevelOfDetail` and backface commands in `ModelFeatureTable`

[j9liu]

Cesium3DTileBatchTable has code for handling tileset._skipLevelOfDetail, which determines whether backface commands are generated for tiles in the tileset. The equivalent class in ModelExperimental ModelFeatureTable, does not handle this yet.

When done, this should be tested by the "create duplicate backface commands" test in Cesium3DTilesetSpec

[j9liu]

This came up on the forum: https://community.cesium.com/t/modelexperimental-rendering-3dtileset-failed/19759/3

[lilleyse]

I started to address that here: https://github.com/CesiumGS/cesium/tree/skip-lods-experimental

The ModelExperimentalDrawCommand refactor was pretty far along and I was about to change how commands are pushed to the command lists to avoid array allocations. If someone else would like to pick that up go ahead!