Sorting and culling ideas

[liaxim]

Tracking Nola's suggestions here:

I think we should group renderdatas by material and transparency sort as part of the cull process. I also think we should use bounding spheres instead of boxes for culling - this would be faster (don't have to regenerate the bounding box, just transform center and radius) but it is less precise. This would be a merge sort during cull which is faster than a large sort after culling is done. The renderer would then visit the individual material groups - opaque first and then transparent. We can use the memory for these lists over and over again each frame.

I am in no way suggesting that you do this as part of this pull request. I want to record these ideas for reform of material processing for the future when we rework the renderer for Vulkan.