SLA gizmo clipping plane resetting

Clipping plane direction is now initialized when the plane is first moved, not when the gizmo is opened. This is how previous versions worked.
This is a minimal-effort solution before the clipping plane is refactored properly.

src/slic3r/GUI/Gizmos/GLGizmoBase.hpp

@@ -220,6 +220,7 @@ public:
 
     float m_clipping_plane_distance = 0.f;
     std::unique_ptr<ClippingPlane> m_clipping_plane;
+    bool m_clipping_plane_was_moved = false;
 
     void stash_clipping_plane() {
         m_clipping_plane_distance_stash = m_clipping_plane_distance;