Replaced the previous KD-tree (for caching points) with a
BVH-tree (thanks to Andre 'jaguarandi' Pinto for help here!).
The bvh is quite a bit faster and doesn't suffer some of the
artifacts that were apparent with the kd-tree.
I've also added a choice of falloff types: Standard, Smooth, and
Sharp. Standard gives a harder edge, easier to see individual
particles, and when used with a larger radius, Smooth and Sharp
falloffs make a much cloudier appearance possible. See the image
below (note the settings and render times too)
http://mke3.net/blender/devel/rendering/volumetrics/pointdensity_bvh.jpg
8622cbca35
