widget: Emit initial style-set signal in the same situations as GTK2

This ensures that widgets that aren't ported and rely on the style-set
signal being emitted work as well as before. They should not rely on
style-set being emitted however.

Note that this function is a no-op if the initial style has been set
already and is very cheap if it has not been set yet. It only becomes
relevant if the resulting style actually gets used.

https://bugzilla.gnome.org/show_bug.cgi?id=639584

gtk/gtksizerequest.c

@@ -339,6 +339,8 @@ compute_size_for_orientation (GtkWidget         *widget,
     {
       gint adjusted_min, adjusted_natural, adjusted_for_size = for_size;
 
+      gtk_widget_ensure_style (widget);
+
       if (orientation == GTK_SIZE_GROUP_HORIZONTAL)
         {
           if (for_size < 0)