Only call render_menu once per frame

An actual surface is not needed to estimate font sizes; a 1x1 image
will do, as long as the cairo context has the same options.

menu.h

@@ -1,6 +1,7 @@
 #ifndef WMENU_MENU_H
 #define WMENU_MENU_H
 
+#include <cairo/cairo.h>
 #include <stdbool.h>
 #include <sys/types.h>
 #include <xkbcommon/xkbcommon.h>
@@ -51,6 +52,10 @@ struct menu {
 
 	struct wl_context *context;
 
+	// 1x1 surface used estimate text sizes with pango
+	cairo_surface_t *test_surface;
+	cairo_t *test_cairo;
+
 	int width;
 	int height;
 	int line_height;