wayland: Set compositor when creating MetaWaylandDataSourceXWayland
create_and_send_dnd_offer() sets the compositor of the offer to the one from the MetaWaylandDataSource. This then later gets used in display_from_offer() when trying to get the context from the compositor. meta_wayland_data_source_xwayland_new() however was not setting the compositor, so this was causing crashes when dragging things from X11 windows on Wayland.
Fixes: 2731f0cd ("wayland: Setup and use ownership chains") Closes: #2723 (closed)