Enable Alt+Tab support in X11 OpenGL fullscreen mode

I would like to present a proposal to allow alt+tab in fullscreen applications using X11 and OpenGL.

The attached diff file is based on version 1.8.3 (13.9.2015), so the line numbers likely don't match the current trunk version. So I also attached my version of the complete file.

The problem

The X11 window is created by setting allow_redirect to true. This overrides the window manager and Alt+Tab does not work. The same holds for the super key etc...

The "not working everywhere" solution

Instead of setting allow_redirect to true, we can change the NET_WM_STATE property. This works with several window managers quite well, but some others behave in a very strange way, when double buffering is enabled (which it is by default). Alt+Tab works, but the window still stays fullscreened in the foreground without having focus.

The working solution

The only thing that seems to work in most environments is to get to windowed mode, when the focus is lost and reactivate it, when the window gets focus again.

The changes

1. delete the line: attributes.override_redirect = CreationParams.Fullscreen;
2. move the call of switchToFullscreen() to a place where the window has already been created
3. on FocusIn: switchToFullscreen(false); // activate
4. on FocusOut: switchToFullscreen(true); // reset
5. extend the switchToFullscreen() method, so it changes the NET_WM_STATE property and sends the respective event

Tested with

Ubuntu, EL 7 and the Desktops MATE, Cinnamon and Gnome 3

This needs to be verified with some more configurations, before it can be safely added to trunk.