GimpTriviallyCancelableWaitable is a proxy object for another
GimpWaitable object, implementing both the GimpWaitable interface
and the GimpCancelable interface.  While waiting on the proxy
simply waits on the underlying waitable, canceling the proxy
doesn't affect the underlying waitable, even if it implements
the GimpCancelable interface as well, but rather causes subsequent
wait operations on the proxy to successfully complete immediately.

This essentially causes cancelation to abort only the wait, rather
than the underlying operation.