scrolledwindow: Resolve whether we use overlay scrollbars early.

scrolledwindow: Resolve whether we use overlay scrollbars early.

Widgets should support size requests before being realized in general,
otherwise this can cause flicker/resize at initial display time as
the toplevel window can make a request before realize/allocate.

This also makes the added testsuite/gtk/scrolledwindow.c test work again,
this was broken because we only ever calculate whether we are going
to use overlay scrollbars once the scrolled window is realized (and
the test case does not realize any window).