scene: Move overlay window resizing to backends

The concept of an overlay window is not universal, it's X11 specific.

src/plugins/platforms/x11/standalone/eglbackend.cpp

@@ -81,9 +81,7 @@ void EglBackend::init()
 
 void EglBackend::screenGeometryChanged(const QSize &size)
 {
-    Q_UNUSED(size)
-
-    // TODO: base implementation in OpenGLBackend
+    overlayWindow()->resize(size);
 
     // The back buffer contents are now undefined
     m_bufferAge = 0;

src/scene.cpp

@@ -69,7 +69,6 @@
 #include "x11client.h"
 #include "deleted.h"
 #include "effects.h"
-#include "overlaywindow.h"
 #include "renderloop.h"
 #include "screens.h"
 #include "shadow.h"
@@ -531,10 +530,7 @@ void Scene::extendPaintRegion(QRegion &region, bool opaqueFullscreen)
 
 void Scene::screenGeometryChanged(const QSize &size)
 {
-    if (!overlayWindow()) {
-        return;
-    }
-    overlayWindow()->resize(size);
+    Q_UNUSED(size)
 }
 
 bool Scene::makeOpenGLContextCurrent()