gl/wayland: fix glib mainloop integration

gl/wayland: fix glib mainloop integration

Implement the prepare and check functions according to the
documentation by returning TRUE when events should be dispatched
via the dispatch function.

As wl_display_read_events never blocks we can call it unconditionally
without looking at the poll status.

This simplifies the implementation and gets rid of a race where the
mainloop could get blocked due to nobody actually reading the events
from the wayland connection.