wallpaper mode is more complicated than setting fullscreen and
level values. Thus, it is implemented in the macosx UI, and we
need to ignore some core events for proper functioning.

close #14151

(cherry picked from commit b8c639dd6c19dd8ecdf0804836c3dd0cf58bb217)
Signed-off-by: David Fuhrmann <dfuhrmann@videolan.org>