mutter would discard the shape region set by the client if its matches
the entire client area in meta_window_x11_update_shape_region().
However, if the window is later resized (maximized or other), the
compositor will fail to update the shape region properly for undecorated
windows because the shape region was discarded, which causes black areas
to appear in place of the updated areas.
If the client window is undecorated, keep the shape region even if when
it matches the client area.
https://gitlab.gnome.org/GNOME/mutter/issues/27Closes: #27
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