Fix 2D debug navigation flickering with tile maps

Both the tile map layers and the debug navigation canvas items did fight for the same z order causing a lot of flickering in certain situations.

Author: smix8

scene/2d/navigation_region_2d.cpp

@@ -452,6 +452,7 @@ void NavigationRegion2D::_update_debug_mesh() {
 	const Transform2D region_gt = get_global_transform();
 
 	rs->canvas_item_set_parent(debug_instance_rid, get_world_2d()->get_canvas());
+	rs->canvas_item_set_z_index(debug_instance_rid, RS::CANVAS_ITEM_Z_MAX - 2);
 	rs->canvas_item_set_transform(debug_instance_rid, region_gt);
 
 	if (!debug_mesh_dirty) {