When scrolled, the container's allocation is smaller than the allocation
of the content. To account for that, commit 2717ca9d08 added the
additional size reported by the layout manager to the content allocation.
However as it did so unconditionally, we now allow children to extend
outside the parent even when *not* scrolled, which breaks any constraints
set on the container (like "width" or "max-height").
Fix this by only extending the child allocation in scrollable dimensions.
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2491
St has the regular abstractions to handle actors that are bigger
than their parent could handle: StScrollable, StScrollView, and
StAdjustment.
However, the only StScrollable implementation available currently
is StBoxLayout, which forces a ClutterBoxLayout as the layout
manager (and relies on it not being unset).
Introduce StViewport, which is a minimal StScrollable implementation
that doesn't rely on any specific layout manager.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/929
