Blender Git Loki

Cycles: Updated the importance metric

The new paper provides an updated importance
metric which is now implemented. Also, a bug
in the index used for background lights in
the MIS calculations have been fixed.

Technical details:
- The new importance metric depends on the
normal at the shading point. The MIS
calculations have been changed
accordingly.

- The uncertainty angle was described in
the paper and is now implemented.

- In the implementation details section
of the paper they mention that they
limit the distance if splitting is
disabled. This is implemented too.