If we have the following sequence:
cache.getImageContent({ filename: "foo", cancellable: cancellable1 });
cache.getImageContent({ filename: "foo", cancellable: cancellable2 });
cancellable1.cancel();
Then the second load will complete with "null" as its content, even though
it was never cancelled, and we'll see a blank image. Meanwhile, since the
second load simply appends to the list of callers for the second load,
cancellable2 does absolutely nothing: cancelling it won't stop the load,
and it will still receive onFinished handling.
To prevent this from happening, give the actual load operation its own
Gio.Cancellable, which is "ref-counted" -- only cancel it when all the other
possible callers cancel.
Based on work from Jasper St. Pierre <jstpierre@macheye.net>
https://bugzilla.gnome.org/show_bug.cgi?id=722149
55d1c7e2ab