Warn when a sub-pass is scheduled or declares attachments

A pass referenced through a SubRenderPass command renders into its
parent's render target: it never produces output of its own, and any
attachment commands it declares are ignored. Both cases previously
failed silently, which made a scene that references a sub-pass from a
RenderOutputProvider render black with no explanation.
With this change we now issue a warning if a SubRenderPass tries to
produce its own render target.

Task-number: QTBUG-148554
Pick-to: 6.12 6.11
Change-Id: I48161d83ec422e5ac3aeb8b6dbfb822fd8c01d72
Reviewed-by: Andy Nichols <andy.nichols@qt.io>