1) The KisBaseRectsWalker now saves notifications for the clones.
   It does this by calculating uncropped changeRect of the source
   layer and storing the node/rect pairs in a special array.
2) KisAsyncMerger takes the contents of this array and notifies the
   clones after the source layer's update is finished
3) If clone needs an area of the source layer that was not generated
   during normal update (it is placed outside the image), it calculates
   its contents in KisUpdateOriginalVisitor by recursively calling to
   KisRefreshSubtreeWalker/KisAsyncMerger.