slic3r: address sanitizer cleanup around wipe tower deletion

When determining whether or not we need to update the object list, we check
all of the objects that are being deleted to see if they're a wipe tower.
Unfortunately, by the time we check, the objects have already been deleted,
and the memory has been freed!  Avoid this by writing down for deleted
objects whether they are wipe towers, and if they were, we can indeed skip
updating the object list.

src/slic3r/GUI/GLCanvas3D.cpp

@@ -2167,15 +2167,16 @@ void GLCanvas3D::reload_scene(bool refresh_immediately, bool force_full_scene_re
 
     struct GLVolumeState {
         GLVolumeState() :
-            volume_idx(size_t(-1)) {}
+            volume_idx(size_t(-1)), was_wipe_tower(0) {}
         GLVolumeState(const GLVolume* volume, unsigned int volume_idx) :
-            composite_id(volume->composite_id), volume_idx(volume_idx) {}
+            composite_id(volume->composite_id), volume_idx(volume_idx), was_wipe_tower(volume->is_wipe_tower) {}
         GLVolumeState(const GLVolume::CompositeID &composite_id) :
-            composite_id(composite_id), volume_idx(size_t(-1)) {}
+            composite_id(composite_id), volume_idx(size_t(-1)), was_wipe_tower(0) {}
 
         GLVolume::CompositeID       composite_id;
         // Volume index in the old GLVolume vector.
         size_t                      volume_idx;
+        bool                        was_wipe_tower;
     };
 
     // SLA steps to pull the preview meshes for.
@@ -2378,8 +2379,21 @@ void GLCanvas3D::reload_scene(bool refresh_immediately, bool force_full_scene_re
             }
         }
         for (int temp_idx = vol_idx; temp_idx < m_volumes.volumes.size() && !update_object_list; temp_idx++) {
-            if (!m_volumes.volumes[temp_idx]->is_wipe_tower)
+            // Volumes in m_volumes might not exist anymore, so we cannot
+            // directly check if they are is_wipe_towers, for which we do
+            // not want to update the object list.  Instead, we do a kind of
+            // slow thing of seeing if they were in the deleted list, and if
+            // so, if they were a wipe tower.
+            bool was_deleted_wipe_tower = false;
+            for (int del_idx = 0; del_idx < deleted_volumes.size(); del_idx++) {
+                if (deleted_volumes[del_idx].volume_idx == temp_idx && deleted_volumes[del_idx].was_wipe_tower) {
+                    was_deleted_wipe_tower = true;
+                    break;
+                }
+            }
+            if (!was_deleted_wipe_tower) {
                 update_object_list = true;
+            }
         }
         for (int temp_idx = vol_idx; temp_idx < glvolumes_new.size() && !update_object_list; temp_idx++) {
             if (!glvolumes_new[temp_idx]->is_wipe_tower)