There were two problems caused by asynchronous nature of vector
layer transformations:

1) Shape layer transformation happens in the GUI thread asynchronously,
   therefore just ask for layer's extent() right after the transformation
   is not enough, this data is not yet ready.

2) Even when requesting shapeLayer->extent() for fetching oldDirtyRect,
   there is a race condition possible. If resetCache() has been called
   quite recently, then the extent will be null. Therefore the patch adds
   a special KisExternalLayer::theoreticalBoundingRect() that is supposed
   to overcome this limitation.

BUG:437886