shell: Fix crash when an input panel is hiding

Set the geometry.dirty on the input panel surface before calling
weston_surface_update_transform, so that the surface gets an output
assigned.
diff --git a/src/shell.c b/src/shell.c
index 2afa1b3..e2715d6 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -2763,6 +2763,7 @@
 		ws = surface->surface;
 		wl_list_insert(&shell->input_panel_layer.surface_list,
 			       &ws->layer_link);
+		ws->geometry.dirty = 1;
 		weston_surface_update_transform(ws);
 		weston_surface_damage(ws);
 		weston_slide_run(ws, ws->geometry.height, 0, NULL, NULL);