Bug in painting undo

[wkjarosz]

How to reproduce:

1. Click on the interface somewhere, like just above a menu bar button
2. drag mouse over image view
3. upon releasing mouse a brush stroke is painted from the previously registered mouse position to the mouse release position
4. history is not updated, so cannot be undo

I suspect the undo state is updated on mouse down on imageview (but it doesn't happen here since the mouse down is on a different UI element). the mouse up still paints onto the image, even though an active painting start command wasn't registered.