Client: Update size hints when updating the decoration

Size hints are in wayland's "window geometry". This includes the space
for any client side decoration that QtWayland has to add.

We need to update the size hints whenever decoration is created or
destroyed and these margins change.

Fixes: QTBUG-121399
Pick-to: 6.7
Change-Id: Ie8c1898859774e1c8dff93695af15327379ad3b6
Reviewed-by: Liang Qi <liang.qi@qt.io>

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -1121,6 +1121,9 @@ bool QWaylandWindow::createDecoration()
         }
         setGeometry(geometry());
 
+        // creating a decoration changes our margins which in turn change size hints
+        propagateSizeHints();
+
         // This is a special case where the buffer is recreated, but since
         // the content rect remains the same, the widgets remain the same
         // size and are not redrawn, leaving the new buffer empty. As a simple