The GTask must be destroyed on the thread that is running its
GMainContext, i.e. the thread that started the task. It must never be
destroyed on the actual task thread when running with
g_task_run_in_thread(), because when it is destroyed, it will unref its
source object and destroy its user data (if a GDestroyNotify was set for
the data using g_task_set_task_data()). The source object and task data
might not be safe to destroy on a secondary thread, though, so this is
incorrect. We have to ensure they are destroyed on the task's context's
thread.

There are different ways we could do this, but the simplest by far is to
ensure the task thread has unreffed the task before the context's thread
executes the callback. And that is simple enough to do using a condition
variable. We have to keep a static global map of all GTasks with
outstanding task threads, which is slightly unfortunate, but we already
have a bunch of global data in this file for managing the thread pool,
and the map will only contain tasks that are currently running in
threads, so it should be small.

Fixes #1346