This patch has two parts:

1) KisShapeLayer should block the updates of the shape-canvas.
   When shapes are added to the layer, they initiate shape
   manager updates. That is not what we want, because all the
   rendered pixel data has already been copied in initShapeLayer()
   call.

2) Add a sanity check in KisDocument::initiateSavingInBackground().
   Theoretically, there should be no pending updates after cloning
   operation. But if they still appear somehow (which is a bug),
   just force them wait until they complete their execution.

BUG:404976
CCBUG:404742