xwayland: Change layer for xwayland override redirect windows

Our positioning of override redirect windows falls apart when an
app is on the fullscreen layer, because we end up putting its
menus and tooltips beneath it. This patch raises the special
override redirect layer to be just below things like on-screen
keyboards (and, unfortunately, above things like panels).

There is no perfect way to deal with this problem, especially
for content like tooltips that don't come with transience hints.

In some cases override redirect menus could be better placed by
using the parenting/transience information provided with them
at map time, and we should probably do that at some point, but
that would still leave us with tooltips below full screen
applications, and the need for this layer change.

based on a patch
Co-authored-by: Hideyuki Nagase <hideyukn@microsoft.com>
Co-authored-by: Steve Pronovost <spronovo@microsoft.com>
Co-authored-by: Brenton DeGeer <brdegeer@microsoft.com>

I changed the layer position and the comments, so:
Signed-off-by: Derek Foreman <derek.foreman@collabora.com>