BSP Entity Bugfixing (Patreon)

I found out that the clipping of BSP entities to the world geometry can crash the engine. While this clipping is responsible for the lack of Z-fighting between BSP entities and the world, it's useless for fixing Z-fighting between different BSP entities.

All things considered, I'm removing this clipping from BSP entities, and adding per-pixel depth tests instead (which alphamasked and alphablended BSP entities already does). This will fix both issues, and may simplify the code a little.