Partially replace convex hull implementation with Bullet implementation

* Bullet's convex hull implementation is significantly more robust
  than the one I implemented, as well as being faster.

* This fixes bug [#32864] "Convex Hull fails in some cases."
  projects.blender.org/tracker/?func=detail&aid=32864&group_id=9&atid=498

  That bug, and others like it, relate to the poor handling of
  co-planar surfaces in the input. Pretty much any model that is
  simple-subdivided a few times gave very bad results before, Bullet's
  implementation handles this much better.

* In order to ensure a smooth transition, the Bullet output is
  translated into the existing HullTriangle hash structure. This makes
  it easy to ensure that the existing slot output stays the same; the
  interactions between the slots are somewhat complicated, detangling
  is a TODO.

* Reviewed by Brecht:
  https://codereview.appspot.com/6741063

source/blender/editors/mesh/mesh_ops.c

@@ -165,7 +165,9 @@ void ED_operatortypes_mesh(void)
 	WM_operatortype_append(MESH_OT_wireframe);
 	WM_operatortype_append(MESH_OT_edge_split);
 
+#ifdef WITH_BULLET
 	WM_operatortype_append(MESH_OT_convex_hull);
+#endif
 
 	WM_operatortype_append(MESH_OT_symmetrize);