xcb: Only create listener window when needed

The xcb QPA plugin uses a separate thread for getting events from the
X11 server in a race-free manner. This thread has to be stopped when
shutting down.  This is done by causing the X11 server to send a special
event that causes the thread to exit when it sees it. (Also, the thread
exits if the xcb connection goes into an error state, but that does not
matter here)

So this event listener window is only needed when Qt is sending events
to itself. This means that it does not have to be kept around and
instead can be created only when needed.

Since this window does not stay around for long, it does not need a
name. However, it does need the list of screens to find the correct root
window, so destroy screens later.

Change-Id: Ib4104cbc3f372fa5d1391bcf5a633a158409862f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Benchmark for QWaitCondition

Change-Id: I89fc0819324c12030bc23ba080b21f3d8d5c9852
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Add C++11 constexpr to 3D vector classes

This turns QVector2D(1,1) into compile-time expressions when using
a C++11 compiler.

Change-Id: Ifea2d1cdfd5481300f9a5a0c7bbe8bfe9115c636
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>