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/CMakeLists.txt

@@ -22,6 +22,7 @@ set(SLIC3R_SOURCES
     AABBTreeLines.hpp
     AABBMesh.hpp
     AABBMesh.cpp
+    AnyPtr.hpp
     BoundingBox.cpp
     BoundingBox.hpp
     BridgeDetector.cpp