Sculpt: Cloth Brush/Filter Collisions

This implements collisions in the solver of the cloth brush/filter. It
uses the scene colliders as a regular physics simulation.

There are still some parameters (friction, distance to the surface...)
that can be exposed as properties in later patches.

Thanks to Sebastian Parborg for helping me with the implementation.

Reviewed By: sergey, zeddb

Differential Revision: https://developer.blender.org/D8019

release/scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -1278,6 +1278,7 @@ class _defs_sculpt:
             layout.prop(props, "cloth_mass")
             layout.prop(props, "cloth_damping")
             layout.prop(props, "use_face_sets")
+            layout.prop(props, "use_collisions")
 
         return dict(
             idname="builtin.cloth_filter",