How Rend3 does lighting and shadows now

[John-Nagle]

Here's an overview of how it works now.

• Directional lights (suns) cast shadows. This works by making a linear pass over every object in the scene in every frame, culling those outside the view frustum, and then doing a depth sort in the CPU. The depth sort, suprisingly, bottlenecks on getting the sort keys, not the sort itself. This may be due to excessive cache misses, since this loop sweeps over so much data.
• There are point lights, but they also require a full pass over all the objects on every frame.
  So, compute cost is (number of lights) x (number of objects). This chokes on big scenes.

Better lighting algorithms are needed. Comments?