Use a fixed, uniform cell size for hair continuum grids.

This is a bit more awkward for artists to use, but necessary for
a stable solution of the hair continuum calculation. The grid size is
defined by the user, the extent of the grid is then calculated based on
the hair geometry. A hard upper limit prevents bad memory allocation
in case too small values are entered.

Conflicts:
	source/blender/physics/intern/BPH_mass_spring.cpp

release/scripts/startup/bl_ui/properties_particle.py

@@ -329,7 +329,7 @@ class PARTICLE_PT_hair_dynamics(ParticleButtonsPanel, Panel):
         # XXX disabled due to stability issues
         #sub.prop(cloth, "pressure", slider=True, text="Pressure")
         #sub.prop(cloth, "pressure_threshold", slider=True, text="Threshold")
-        col.prop(cloth, "voxel_resolution")
+        col.prop(cloth, "voxel_cell_size")
 
         split.separator()