Adjusts view after mouse/border selection if some selected items are out of view bounds.
To get as much of the selection into view as possible, this adjusts view first for the last, then for the first element in the selection.
Also, if region is pretty small, view adjustment is skipped, as otherwise the view is focused on the first element only, which isn't really useful IMHO.
Maybe not so nice: Since we do two view alignment iterations, UI_view2d_curRect_validate, which is a rather big function *might* be called twice under certain circumstances (border select & total size of selected elements is exceeds view bounds). I think that's totally acceptable though.
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