Todo issue: sculpting on deformed mesh

Used a crazyspace approach (like in edit mode), but only modifiers with
deformMatricies are allowed atm (currently shapekeys and armature modifiers only).
All the rest modifiers had an warning message that they aren't applied because
of sculpt mode. Deformation of multires is also unsupported.

With all this restictions users will always see the actual "layer" (or maybe
mesh state would be more correct word) they are sculpting on.

Internal changes:
- All modifiers could have deformMatricies callback (the same as deformMatriciesEM but
  for non-edit mode usage)
- Added function to build crazyspace for sculpting (sculpt_get_deform_matrices), but it
  could be generalized for usage in other painting modes (particle edit mode, i.e)

Todo:
- Implement crazyspace correction to support all kinds of deformation modifiers
- Maybe deformation of multires isn't so difficult?
- And maybe we could avoid extra bad-level-stub for ED_sculpt_modifiers_changed
  without code duplicating?

source/blender/editors/include/ED_sculpt.h

@@ -40,6 +40,7 @@ void ED_operatortypes_sculpt(void);
 void sculpt_get_redraw_planes(float planes[4][4], struct ARegion *ar,
 				   struct RegionView3D *rv3d, struct Object *ob);
 void ED_sculpt_force_update(struct bContext *C);
+void ED_sculpt_modifiers_changed(struct Object *ob);
 
 /* paint_ops.c */
 void ED_operatortypes_paint(void);