Wayland: fix slider rendering underneath fullscreen windows

[lukefromdc]

Set slider as the top layer not the bottom using gtk-layer-shell Fix https://github.com/mate-desktop/mate-media/issues/198

[Sunderland93]

Thanks, it works!

[lukefromdc]

Merging as this is literally a one word fix (correcting an error on my part) and has been confirmed by the reporter of the issue to work