macOS: Flush sublayers via separate IOSurface backingstores

Flushing sublayers via QImage copies of the root IOSurface was causing
performance regressions due to the constant allocations of new images
each frame.

We now re-use the QCALayerBackingStore implementation for sublayers,
which gives a dynamic swap-chain.

We're still paying the CPU cost of the copy from the root backingstore
to the layered backingstores, as well as the memory cost, but at least
improves the situation.

We do not try to be smart and paint directly into the sublayers,
as that would leave the root backingstore stale, potentially causing
glitches when views are repositioned. Investigating this is left
for future work.

Fixes: QTBUG-82986
Change-Id: I758a3d8e1e40e2ed4fe6bc590a4a5a988d87a3a7
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit ce2d68ebe1aefeae78ff2fd8ec5ff7e20790ef69)

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