go back to OpenLayers5 for now, keep the new LayerSwitcher

Once the labels work correctly, going back to OpenLayers6 will be
trival.

html/script.js

@@ -2313,7 +2313,7 @@ function toggleMapDim(switchOn) {
 		ol.control.LayerSwitcher.forEachRecursive(layers_group, function(lyr) {
 			if (lyr.get('type') != 'base')
 				return;
-			lyr.dimKey = lyr.on('postrender', dim);
+			lyr.dimKey = lyr.on('postcompose', dim);
 		});
 
 		$('#mapdim_checkbox').addClass('settingsCheckboxChecked');