Limit directional shadow map frustum to scene bounds

If using scene bounds creates a smaller shadow map camera frustum, use
that.

Task-number: QTBUG-83362
Change-Id: I77b261cfe07aca9a238b907ece842d6f5266a395
Reviewed-by: Antti Määttä <antti.maatta@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>