Move XdgSurfaceClient::updateDepth to WaylandClient

It is strictly related to the surface and this way it can be used from
elsewhere.

waylandclient.cpp

@@ -14,6 +14,7 @@
 #include <KWaylandServer/display.h>
 #include <KWaylandServer/clientconnection.h>
 #include <KWaylandServer/surface_interface.h>
+#include <KWaylandServer/buffer_interface.h>
 
 #include <QFileInfo>
 
@@ -273,4 +274,13 @@ void WaylandClient::doSetActive()
     }
 }
 
+void WaylandClient::updateDepth()
+{
+    if (surface()->buffer()->hasAlphaChannel() && !isDesktop()) {
+        setDepth(32);
+    } else {
+        setDepth(24);
+    }
+}
+
 } // namespace KWin

waylandclient.h

@@ -37,6 +37,7 @@ public:
     void killWindow() override;
     QByteArray windowRole() const override;
 
+    void updateDepth();
     void setCaption(const QString &caption);
 
 protected:

xdgshellclient.cpp

@@ -483,15 +483,6 @@ void XdgSurfaceClient::updateGeometryRestoreHack()
     }
 }
 
-void XdgSurfaceClient::updateDepth()
-{
-    if (surface()->buffer()->hasAlphaChannel() && !isDesktop()) {
-        setDepth(32);
-    } else {
-        setDepth(24);
-    }
-}
-
 QRect XdgSurfaceClient::frameRectToBufferRect(const QRect &rect) const
 {
     const int left = rect.left() + borderLeft() - m_windowGeometry.left();

xdgshellclient.h

@@ -99,7 +99,6 @@ private:
     void resetHaveNextWindowGeometry();
     QRect adjustMoveResizeGeometry(const QRect &rect) const;
     void updateGeometryRestoreHack();
-    void updateDepth();
     void internalShow();
     void internalHide();
     void cleanGrouping();