Added basic filtering feature for velocity smoothing.

This is part of the original method from "Volumetric Methods for
Simulation and Rendering of Hair". The current filter is a simple box
filter. Other energy-preserving filters such as gaussian filtering
can be implemented later.

The filter size is currently given as a cell count. This is not ideal,
rather it should use a geometrical length value, but this is too
abstract for proper artistical use. Eventually defining the whole grid
in terms of spatial size might work better (possibly using an external
object).

release/scripts/startup/bl_ui/properties_particle.py

@@ -323,6 +323,7 @@ class PARTICLE_PT_hair_dynamics(ParticleButtonsPanel, Panel):
         sub.separator()
         
         sub.prop(cloth, "voxel_resolution")
+        sub.prop(cloth, "voxel_filter_size")
 
         col = split.column()
         col.label(text="Damping:")