Set a size in qopenglwindow example

Use showMaximized(). Just calling show() without setting a size can result
in a zero-sized, invisible window on some platforms.

Change-Id: Ifa48258060e3d651c2fac3a1409a26a2c3db6bdb
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>

examples/opengl/qopenglwindow/main.cpp

@@ -185,7 +185,7 @@ int main(int argc, char **argv)
     fmt.setDepthBufferSize(24);
     fmt.setStencilBufferSize(8);
     window.setFormat(fmt);
-    window.show();
+    window.showMaximized();
 
     return app.exec();
 }