On destroying wl_egl_window, remove reference in WaylandNativeWindow

On some cases, wl_egl_window could be destroyed when WaylandNativeWindow
had still alive references. In this case, the code in WaylandNativeWindow
that detachs the reference to wl_egl_window will access already freed
memory. This was reported by valgrind.
Merge pull request #242 from jdapena/wl_egl_window_destroy_issue

On destroying wl_egl_window, remove reference in WaylandNativeWindow