This allows removing a bunch of internal faces, improving frametime greatly when looking at a distant forest.
fixes#579
makes #823 more important, since this doubled the cost
I gave up on two restrictions:
1. low-to-high LOD borders are no longer prevented. This was only a problem for low render distance + fast movement, and the consequences of the fix were causing flickering in some cases, and to be honest I prefer small gaps in the terrain.
2. 2×2×2 chunks are grouped and rendered together. This makes the vertex shader a bit simpler and allows hierarchical traversal of chunks. However this means that the minimume render distance had to be increased again.
Overall this gave around a 30% reduction in CPU time for selecting the visible chunks. This also reduced jitter during loading.
makes #722 less severe
would make #745 more effective
Further optimizations might be possible.
Slightly slower, but halfs GPU memory usage and allows storing 3× more faces on the GPU.
Also makes meshing a bit slower, but transparency sorting should be a lot faster, since it's shuffling around less memory.
