stacking: Use WIN_LAYER_FULLSCREEN for focused fullscreen windows when modifying _NET_WM_STATE_ABOVE.

If a normal window is created with _NET_WM_STATE_ABOVE and _NET_WM_STATE_FULLSCREEN, and then
_NET_WM_STATE_ABOVE gets removed. The window should still reside above of WIN_LAYER_DOCK instead of
reset to initial_layer, which is WIN_LAYER_NORMAL and below WIN_LAYER_DOCK.

According to the recommended stacking order specified in EWMH, focused fullscreen windows should be
on top of other windows, but cdcce736

 explicitly changed that so I didn't pursue full EWMH conformance.

Fix some games such as Deus Ex: Game of the Year Edition and The Witcher 3 having their fullscreen
windows behind xfce4-panel after changing resolution in-game when running on Proton.

Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>