icon factory: Add gtk_icon_set_render_icon_surface

Support scales when falling back to loading icons from the
icon theme.

In order to actually render scaled icons we add
gtk_icon_set_render_icon_surface which renders to a cairo_surface_t
which includes whatever scaling you need for scaled icons.

gtk/gtkstylecontext.h

@@ -885,6 +885,13 @@ GDK_AVAILABLE_IN_ALL
 GdkPixbuf  * gtk_icon_set_render_icon_pixbuf   (GtkIconSet      *icon_set,
                                                 GtkStyleContext *context,
                                                 GtkIconSize      size);
+GDK_AVAILABLE_IN_3_10
+cairo_surface_t  *
+gtk_icon_set_render_icon_surface               (GtkIconSet      *icon_set,
+						GtkStyleContext *context,
+						GtkIconSize      size,
+						int              scale,
+						GdkWindow       *for_window);
 
 GDK_AVAILABLE_IN_ALL
 void        gtk_style_context_set_screen (GtkStyleContext *context,