x11: Fix compositing status detection
When finishing compositing, Workspace::compositing() will return true, but it will be preferred if it returns false instead so kwin can properly update x11 window visibility status (in X11Client::updateVisibility()), etc.
Instead of checking whether the compositor has a scene, check the status of the compositor. It will not be "On" during teardown.
Edited by Vlad Zahorodnii