Calling XInitThreads before the QApplication constructor makes the constructor block

The constructor hangs waiting for a mutex, within a call to the XInput extension. This is a bug in Xlib or X11, but Qt should allow to turn off the XInput call without requiring a complete reconfigure or recompile, ie. through an environment variable.

XInitThreads needs to be called to be able to render OpenGL from multiple threads, which works without problems on Mac and Windows.