Support opaque 16-bit images as RGB30 with Wuffs

Do not check whether the window's visual can make use of them,
since they're arguably rare enough.

With transparent images, we're limited by Cairo's formats.

fastiv-view.c

@@ -181,6 +181,10 @@ fastiv_view_realize(GtkWidget *widget)
 	gtk_widget_register_window(widget, window);
 	gtk_widget_set_window(widget, window);
 	gtk_widget_set_realized(widget, TRUE);
+
+	// Without the following call, or the rendering mode set to "recording",
+	// RGB30 degrades to RGB24.
+	gdk_window_ensure_native(window);
 }
 
 static gboolean