Blender Git Loki

Cycles: Implemented the SAOH splitting method

Previously the light bvh construction code used a simple method
to split a node into two child nodes based only on the bounding
box of the node. Now, it splits a node into two by looking at
the energy and direction of the lights inside the nodes and the
area of their bounding boxes.

Technical changes:
- Implemented orientation BVH build
- Added energy to each node based on emission
- Updated PDF calculations for lights when using the light BVH