SLA backend thread-safety improvements

- Put AnyPtr into separate header, it deserves one
- Add means to handle shared pointers inside AnyPtr
- Use shared pointers in sla csg collection for meshes not owned by Model
- Add method to get the last completed step in PrintObjectBase
- Make SLAPrintObject::get_parts_to_slice() safe to call from UI thread against background thread activity.

src/libslic3r/SLAPrint.hpp

@@ -123,16 +123,9 @@ public:
     // like hollowing and drilled holes.
     const TriangleMesh & get_mesh_to_print() const;
 
-    const Range<CSGContainer::const_iterator> get_parts_to_slice() const
-    {
-        return range(m_mesh_to_slice);
-    }
+    std::vector<csg::CSGPart> get_parts_to_slice() const;
 
-    const Range<CSGContainer::const_iterator> get_parts_to_slice(SLAPrintObjectStep step) const
-    {
-        auto r = m_mesh_to_slice.equal_range(step);
-        return {r.first, r.second};
-    }
+    std::vector<csg::CSGPart> get_parts_to_slice(SLAPrintObjectStep step) const;
 
     sla::SupportPoints      transformed_support_points() const;
     sla::DrainHoles         transformed_drainhole_points() const;
@@ -373,6 +366,15 @@ private:
     // Holds CSG operations for the printed object, prioritized by print steps.
     CSGContainer                  m_mesh_to_slice;
 
+    auto mesh_to_slice(SLAPrintObjectStep s) const
+    {
+        auto r = m_mesh_to_slice.equal_range(s);
+
+        return Range{r.first, r.second};
+    }
+
+    auto mesh_to_slice() const { return range(m_mesh_to_slice); }
+
     // Holds the preview of the object to be printed (as it will look like with
     // all its holes and cavities, negatives and positive volumes unified.
     // Essentially this should be a m_mesh_to_slice after the CSG operations