wayland: Initialize Window::depth to 32

Window::depth and Window::hasAlpha make no sense on Wayland. The main
reason why we can't rid of them completely yet are X11Window and
Unmanaged.

This change makes WaylandWindow initialize depth to 32 by default to
make wayland window subclasses less boilerplaty.

src/inputpanelv1window.cpp

@@ -182,7 +182,6 @@ void InputPanelV1Window::moveResizeInternal(const QRectF &rect, MoveResizeMode m
 
 void InputPanelV1Window::handleMapped()
 {
-    updateDepth();
     maybeShow();
 }
 

src/layershellv1window.cpp

@@ -250,7 +250,6 @@ void LayerShellV1Window::handleUnmapped()
 void LayerShellV1Window::handleCommitted()
 {
     if (surface()->buffer()) {
-        updateDepth();
         setReadyForPainting();
     }
 }

src/waylandwindow.cpp

@@ -37,6 +37,7 @@ Q_DECLARE_FLAGS(WaylandGeometryTypes, WaylandGeometryType)
 WaylandWindow::WaylandWindow(SurfaceInterface *surface)
 {
     setSurface(surface);
+    setDepth(32);
     setupCompositing();
 
     connect(surface, &SurfaceInterface::shadowChanged,
@@ -220,15 +221,6 @@ void WaylandWindow::doSetActive()
     }
 }
 
-void WaylandWindow::updateDepth()
-{
-    if (surface()->buffer()->hasAlphaChannel()) {
-        setDepth(32);
-    } else {
-        setDepth(24);
-    }
-}
-
 void WaylandWindow::cleanGrouping()
 {
     if (transientFor()) {

src/waylandwindow.h

@@ -36,7 +36,6 @@ public:
     virtual QRectF frameRectToBufferRect(const QRectF &rect) const;
     bool isHidden() const;
 
-    void updateDepth();
     void setCaption(const QString &caption);
 
 protected:

src/xdgshellwindow.cpp

@@ -175,7 +175,6 @@ void XdgSurfaceWindow::handleCommit()
     m_lastAcknowledgedConfigureSerial.reset();
 
     setReadyForPainting();
-    updateDepth();
 }
 
 void XdgSurfaceWindow::handleRolePrecommit()