scrolledwindow: Better size requisition for GTK_SCROLL_NATURAL children

scrolledwindow: Better size requisition for GTK_SCROLL_NATURAL children

GtkScrolledWindow leans towards using the minimum size of its child
widget, unless the scrollbar policy is GTK_POLICY_NEVER. This is
probably fine for most GtkScrollable implementations out there.
Especially when using GTK_SCROLL_MINIMUM, which is the default for all
implementations inside gtk+.

However, this is not good for GTK_SCROLL_NATURAL children. eg.,
VteTerminal's minimum size is 1x1 and natural size is the number of
visible rows and columns requested by the user. We really want to use
the natural size unless the user has resized the window to change that.

https://bugzilla.gnome.org/show_bug.cgi?id=766569