clients/confine: Reset motion lines on Backspace

Clear the white lines that is drawn by pointer motions. It makes it
easier to debug pointer movements as one won't need to restart confine
just to get a clean plate.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>

clients/confine.c

@@ -192,6 +192,11 @@ key_handler(struct window *window, struct input *input, uint32_t time,
 	case XKB_KEY_Escape:
 		display_exit(confine->display);
 		break;
+	case XKB_KEY_BackSpace:
+		cairo_surface_destroy(confine->buffer);
+		confine->buffer = NULL;
+		window_schedule_redraw(confine->window);
+		break;
 	}
 }