Debarshi Ray authored Dec 03, 2018 and Philip Withnall committed Feb 06, 2019

Once cancelled, a GTask's callback should not only be invoked
asynchronously with respect to the creation of the task, but also with
respect to the GCancellable::cancelled handler. This is particularly
relevant in cases where the cancellation happened in the same thread
where the task is running.

Spotted by Dan Winship and Michael Catanzaro.

Closes #1608