Weird NVIDIA behavior fixup part 2. Now that we do no longer freeze when
NVIDIA decides to create an OpenGL error on startup
(aefb5f4dd9d41aa7377d56ece203089c73aefe07), we experience a new issue.
KWin is terminating (no idea why, ) and at the same time the OpenGL freeze
protection thread is still running. So far we did not terminate the
thread on shutdown and thus we hit an abort in Qt.
This change ensures that we properly terminate the thread on shutdown.
 My current theory is that games terminate KWin, common pattern of
bug reports is "steam".