Instead of taking resource and send/cancel funcs, take a
MetaWaylandDataSource, which exposes all the vfuncs to do the same on the
internal resource.
This has the added side effect that only MetaWaylandDataSource has a
pointer to the wl_resource, which may be unset untimely.
Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/842
This object represents a Wayland selection owner. In order to invert the
FD direction (we hand an output fd, but want an inpu fd), create an
intermediate pipe so we can then create a GInputStream on top of it.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/320
