gg: cleanup the minimal gg program more

examples/gg/polygons.v

@@ -11,7 +11,6 @@ fn main() {
 		window_title: 'Polygons'
 		frame_fn: frame
 	)
-	context.user_data = context
 	context.run()
 }
 

vlib/gg/gg.c.v

@@ -179,6 +179,9 @@ pub fn new_context(cfg Config) &Context {
 		ui_mode: cfg.ui_mode
 		native_rendering: cfg.native_rendering
 	}
+	if isnil(cfg.user_data) {
+		g.user_data = g
+	}
 	g.set_bg_color(cfg.bg_color)
 	// C.printf('new_context() %p\n', cfg.user_data)
 	window := C.sapp_desc{