Track the fd indices we sent to Pinos and free the associated buffer
when Pinos sends us release-fd. Only do this for non tempfile fds, as we
made the tmpfile ones ourselves and we're not going to reuse them.
Add instrospection of client and source-output.
Add properties to source-output and to CreateSourceOutput/Input
Add helper to fill properties of context.
Add client-name to pinossrc and pinossink
Improve test-subscribe to show all new introspection details.
Add a mainloop wrapper that runs the mainloop in a separate thread and
has some synchronization primitives.
Use new mainloop in gstreamer source and sink elements
