Cycles Hair: Strand Minimum Pixel Size

Code is added to restrict the pixel size of strands in cycles. It works best with ribbon primitives and a preset for these is included. It uses distance dependent expansion of the strands and then stochastic strand removal to give a fading. To prevent a slowdown for triangle mesh objects in the BVH an extra visibility flag has been added. It is also only applied for camera rays.

The strand width settings are also changed, so that the particle size is not included in the width calculation. Instead there is a separate particle system parameter for width scaling.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56072 954f8c5b-7b00-dc11-b283-0030488c597c