Castle Game Engine: Big renderer improvements: Correct and automatic blending sorting, more powerful batching (now cross-scene), easier and more reliable occlusion culling and occlusion sorting

It’s a pleasure to finally close a big TODO from our roadmap. This refactor took 122 commits, and it reworks a core rendering code in the engine. This was hard to do but also was a lot of fun — I used this opportunity to do a lot of code simplifications. These simplifications also translate …