Implemented internal hair pressure to prevent hair from collapsing in

on itself.

This uses the same voxel structure as the hair smoothing algorithm.
A slightly different method was suggested in the original paper
(Volumetric Methods for Simulation and Rendering of Hair), but this is
based on directing hair based on a target density, which is another
way of implementing global goals. Our own approach is to define a
pressure threshold above which the hair is repelled in the density
gradient direction to simulate internal pressure from collisions.

release/scripts/startup/bl_ui/properties_particle.py

@@ -316,6 +316,8 @@ class PARTICLE_PT_hair_dynamics(ParticleButtonsPanel, Panel):
         sub.prop(cloth, "bending_stiffness", text="Bending")
         sub.prop(cloth, "internal_friction", slider=True)
         sub.prop(cloth, "collider_friction", slider=True)
+        sub.prop(cloth, "pressure", slider=True)
+        sub.prop(cloth, "pressure_threshold", slider=True)
 
         col = split.column()
         col.label(text="Damping:")