Blender Git Loki

Cycles: Tweak to leaf creation criteria in all BVH types

Since leaf node gets split further into per-primitive type leaves old check
for number of curves became a bit ridiculous -- it might lead to two leaf nodes
each of which would contain only one curve primitive (one motion curve and one
regular curve).

This lead to quite dramatic slowdown for Victor model -- around 40%, which is
totally unacceptable.

This commit is aimed to prevent such situation and from quick render test it
seems victor is now back to normal render time. Further testing is needed tho.

There are also other ideas about splitting the node, will need to look into
them next.