Cycles: add "Transparent Shadow" option for materials, to disable them per material.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57542 954f8c5b-7b00-dc11-b283-0030488c597c
Cycles: optimization for BVH traveral on CPU's with SSE3, using code from Embree.

On the BMW scene, this gives roughly a 10% speedup overall with clang/gcc, and 30%
speedup with visual studio (2008). It turns out visual studio was optimizing the
existing code quite poorly compared to pretty good autovectorization by clang/gcc,
but hand written SSE code also gives a smaller speed boost there.

This code isn't enabled when using the hair minimum width feature yet, need to
make that work with the SSE code still.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57543 954f8c5b-7b00-dc11-b283-0030488c597c