Fix flickering with Qt 5.12

Summary:
Mesa requires XESetWireToEvent xlib callbacks to be called
when DRI2 is used. This is done by the GLX integration in
the Qt's xcb plugin, but Qt 5.12 initializes the GLX integration
only when required, e.g. when a window with OpenGL support is
created or when availability of OpenGL is checked.

So force initialization of the GLX integration by calling
QOpenGLContext::supportsThreadedOpenGL().

https://codereview.qt-project.org/#/c/6557/
https://bugzilla.opensuse.org/show_bug.cgi?id=1120090

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: davidedmundson, graesslin, fvogt, filipf, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18366