macOS: Optionally flush sub-layers via sub-image copies of the backingstore

When we're flushing the backingstore to sub-views with their own layers
we don't want to pay the cost of uploading the whole backingstore to the
GPU in the case where we're dealing with a discrete GPU.

To work around this we make a copy of the appropriate part of the surfcace.

This results in additional copies of the data, and will need further
investigation to limit these.

Task-number: QTBUG-77447
Change-Id: I318ae80e433dd7b0a55fd5a598b19f114d8bd28e
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>