Relevant since rB6bc6d016c5e7

Scrolling to an item after opening relevant parents can go wrong if said
parent e.g. the last in the list [as in: then the Outliner does not
scroll down all the way]
It stems from the fact that 'region->v2d.tot.ymin' is not up-to-date in
outliner_scroll_view after outliner_show_active opens up parents, 'tot'
will only update on a redraw.

Now calculate the trees height on the fly using
'outliner_tree_dimensions()'.

ref D9521
ref T82553