More Overdraw Optimizations Progress (Patreon)

Started optimizing the overdraw of BSP entities. Now some maps that were previously unplayable have become playable, and in some cases the performance is over 100% faster than in version 0.17.1.

There still are significant performance issues to figure out, and maps with little to no overdraw (e.g. vanilla Quake maps) won't benefit from these optimizations, of course.