Navmesh should not generate on backfaces of geometry

Generating navmesh on backfaces causes a lot of problems-- the clearest example would be a ceiling, where currently, navmesh gets generated **above** the room.

By default, Sketchfab uploads have backfaces turned ON, and it seems most users do not turn this off. One reason might be because those backfaces cast shadows and the result looks better and doesn't require making separate shadow-casting geometry when using lights with shadows, such as sunlight. However, this makes the NavMesh generation create a lot of unwanted traversible areas.

Locate the navmesh generation entry point and inspect how geometry faces are selected, especially for backfaces and ceilings. Reproduce the issue with a room containing a ceiling, then verify that generated navmesh covers only intended traversable surfaces and no longer appears above the room.

Written by the indexing model from the issue text.