Close bld/OSG floor: OSG 3.4.1 from source + port osgQt to Qt6 + wire BgViewer dtEntity

Migrate BuildingViewer (16 files: DDS members, m_logger inits, QOpenGLWidget); gui:3:bld.info + gui:3:bdv now load

qt6-after/qt6-migrated-legatus/Lts/Plugins/BuildingViewer/Common/bdv_helper.cpp

@@ -22,7 +22,8 @@ int32_t featureId2DatabaseId(const QString &strFeatureId)
   if (!strFeatureId.startsWith(gStrFeatureIdPrefix))
     return -1;
   bool isOk = false;
-  int32_t buildingDatabaseId = strFeatureId.midRef(gStrFeatureIdPrefix.size()).toInt(&isOk);
+  // Qt6: QString::midRef removed; mid() returns the same substring by value.
+  int32_t buildingDatabaseId = strFeatureId.mid(gStrFeatureIdPrefix.size()).toInt(&isOk);
   return isOk ? buildingDatabaseId : -1;
 }
 }