Hollowing: allow to place drain holes on the inner surface of a hollowed cavity

Drain holes are not allowed to be placed on its own inner surface. This was recently (0e3ebb3)
done by forcing invalidation of the slaposDrillHoles, which also invalidates and therefore
hides the drilled mesh. However, that also hides the hollowed mesh and it is not possible
to place holes inside the cavity.

This change does not dump the drilled mesh, but checks that no raycast hit ends up in a hole.

src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.cpp

@@ -408,7 +408,14 @@ bool GLGizmoSlaSupports::unproject_on_mesh(const Vec2d& mouse_pos, std::pair<Vec
     // The raycaster query
     Vec3f hit;
     Vec3f normal;
-    if (m_c->m_mesh_raycaster->unproject_on_mesh(mouse_pos, trafo.get_matrix(), camera, hit, normal, m_c->m_clipping_plane.get())) {
+    if (m_c->m_mesh_raycaster->unproject_on_mesh(
+            mouse_pos,
+            trafo.get_matrix(),
+            camera,
+            hit,
+            normal,
+            m_c->m_clipping_plane_distance != 0.f ? m_c->m_clipping_plane.get() : nullptr))
+    {
         // Check whether the hit is in a hole
         bool in_hole = false;
         // In case the hollowed and drilled mesh is available, we can allow