Previously when using light cache, there could be artifacts caused when
voxel points that were sampled outside the volume object's geometry got
interpolated into the rest of the volume. This commit adds a (similar
to a dilate) filter pass after creating the light cache, that fills
these empty areas with the average of their surrounding voxels.
http://mke3.net/blender/devel/rendering/volumetrics/vol_lightcache_filter.jpg