Implemented loading of hollowed SLA mesh into the scene

in place of the normal mesh.

WIP: The code is inefficient (it does not store the timestamp
of the hollowed mesh, therefore it refreshes the hollowed mesh on each
scene update) and if the hollowing gets invalidated, the original mesh
is currently not being reloaded and the hollowed mesh is still visible.

src/libslic3r/SLAPrint.cpp

@@ -1103,6 +1103,8 @@ const ExPolygons &SliceRecord::get_slice(SliceOrigin o) const
 bool SLAPrintObject::has_mesh(SLAPrintObjectStep step) const
 {
     switch (step) {
+    case slaposHollowing:
+        return m_hollowing_data && !m_hollowing_data->hollow_mesh_with_holes.empty();
     case slaposSupportTree:
         return ! this->support_mesh().empty();
     case slaposPad: