gdk/wayland: use the multi-thread safe wayland API

gdk/wayland: use the multi-thread safe wayland API

This is required for proper integration with any other library/application that
may perform wayland API calls and poll() the wayland fd from multiple threads.
Using wl_display_dispatch{_queue}() is thread-safe if not mixed with custom
poll() usage, which GSource/GMainContext does.

Essentially, the problem is that multiple threads polling and reading
the same fd is extremely racy.  Use the wayland provided API for allowing
concurrent access to the wayland display fd.

See the wayland man pages for wl_display_prepare_read(),
wl_display_cancel_read() and wl_display_read_events() for more details.

https://bugzilla.gnome.org/show_bug.cgi?id=763852