Fix layer data loss when pressing Esc multiple times, while transforming stroke
There were several problems:
- When the jobs from m_overriddenCommand have been executed, they shouldn't be added to the strokes's undo commands queue. After the clear-selection job, the paint device gets new transaction, therefore all the redo information is lost. And commands from the previous stroke are not valid anymore.
- Since the commands from m_overridenCommand do not take part in normal cancel/undo process, in case of stroke cancellation we should re-apply them manually. And therefore, we must ensure that clear selection and create-preview-device actions are executed before cancellation action is performed. Therefore the patch introduces a special isCancellable() tag of the stroke jobs.
- Since finishStrokeCallback() now adds more jobs to the strokes queue, we need some way to mark them non-cancellable. It is done by the same isCancellable() tag.