Ensure the Compositor does not start during termination

Summary:
The SceneOpenGLES test was starting to randomly crash on my system. On
investigation I observed that there was a graphics reset ("Attempting to
reset compositing.") which triggered a delayed restart of the
compositor. This even was only processed while waiting for XWayland to
terminate. Which resulted in a crash in KWin::getXServerVersion as the
xcb connection broke.

It makes no sense to setup the compositor again during application
shutdown. Thus the dtors set a flag that they are terminating the
application and Compositor::setup is checking for it.

Test Plan: Test no longer crashes, although it goes through the crashing path

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18015