While the default implementation will spawn a thread per new
pushed task, this new implementation instead spawns a maximum
number of threads, then queues new tasks on existing threads.
The thread that the new task will be queued on is picked in
a pretty naive fashion, by simply popping the first thread
from a queue and pushing it back to the tail, but this is
an implementation detail and can always be sophisticated
in the future if the need arises.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/692>
Since we started depending on GLib 2.44, we can be sure this macro is
defined (it will be a no-op on compilers that don't support it). For
plugins we should just start using `G_DECLARE_FINAL_TYPE` which means
we no longer need the macro there, but for most types in core we don't
want to break ABI, which means it's better to just keep it like it is
(and use the `#ifdef` instead).
We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
Remove the static function set on the TaskPool before _prepare() is called and
allow for assigning a function to a Task when we _push().
Update the examples
