Fix place on bed and sinking instances:

all sinking objects were incorrectly shifted to bed when
'place on face' was applied to any object.

src/slic3r/GUI/Gizmos/GLGizmoFlatten.cpp

@@ -42,6 +42,8 @@ std::string GLGizmoFlatten::on_get_name() const
 
 bool GLGizmoFlatten::on_is_activable() const
 {
+    // This is assumed in GLCanvas3D::do_rotate, do not change this
+    // without updating that function too.
     return m_parent.get_selection().is_single_full_instance();
 }