Call setupOverlay also from GLES backend

This fixes my issue that mouse events are eaten on R600G.
I am a little bit surprised that it worked at all with nouveau.
Now my issues are fixed and finally, finally my primary system uses
the free driver stack (though only GLES).

scene_opengl_egl.cpp

@@ -99,6 +99,8 @@ bool SceneOpenGL::initRenderingContext()
     if (!wspace->createOverlay()) {
         kError(1212) << "Could not get overlay window";
         return false;
+    } else {
+        wspace->setupOverlay(None);
     }
     surface = eglCreateWindowSurface(dpy, config, wspace->overlayWindow(), 0);