Use GTK_RESIZE_PARENT resize_mode for GtkViewport

We used to use GTK_RESIZE_QUEUE, but that is problematic for e.g
a GtkScrolledWindow with NEVER scroll policies, as size changes
in ancestors will never get propagated to the scrolled window, causing
it to not have the correct size.

This is a slight performance hit, but in practice its not bound to be
problematic. In typical UIs there is only a single "large" GtkScrolledWindow
visible at a time, so a size requeust propagating out of such a window
will only hit the smaller amount of widgetry outside the scrolled window,
and additionally all such widgets will have their size request caches
still valid.

https://bugzilla.gnome.org/show_bug.cgi?id=690099