explicitly use 'event' in onPointerUp (#24708)

examples/webgl_geometry_spline_editor.html

@@ -363,7 +363,7 @@
 
 			}
 
-			function onPointerUp() {
+			function onPointerUp( event ) {
 
 				onUpPosition.x = event.clientX;
 				onUpPosition.y = event.clientY;