ENH: select a volume in assembly view can enter painting

In assembly view, default selection is volume and painting needs an
object. It's inconvenient to select a full object to get in painting.

Jira: STUDIO-3322

Change-Id: I980e5a3ca7a9c5cf2800f28a38c1c68b978a343b

src/slic3r/GUI/Gizmos/GLGizmoMmuSegmentation.cpp

@@ -54,7 +54,8 @@ bool GLGizmoMmuSegmentation::on_is_selectable() const
 
 bool GLGizmoMmuSegmentation::on_is_activable() const
 {
-    return GLGizmoPainterBase::on_is_activable() && wxGetApp().filaments_cnt() > 1;
+    const Selection& selection = m_parent.get_selection();
+    return !selection.is_empty() && (selection.is_single_full_instance() || selection.is_any_volume()) && wxGetApp().filaments_cnt() > 1;
 }
 
 //BBS: use the global one in 3DScene.cpp

src/slic3r/GUI/Gizmos/GLGizmosCommon.cpp

@@ -117,7 +117,9 @@ bool CommonGizmosDataPool::check_dependencies(CommonGizmosDataID required) const
 void SelectionInfo::on_update()
 {
     const Selection& selection = get_pool()->get_canvas()->get_selection();
-    if (selection.is_single_full_instance()) {
+    // BBS still keep object pointer when selection is volume
+    //if (selection.is_single_full_instance()) {
+    if (!selection.is_empty()) {
         m_model_object = selection.get_model()->objects[selection.get_object_idx()];
         m_z_shift = selection.get_volume(*selection.get_volume_idxs().begin())->get_sla_shift_z();
     }