[wayland] Ensure to not call into x11 on Compositor tear-down

During Compositor tear down Xwayland is already destroyed. Thus it
doesn't make sense to try to delete the support properties: either
it freezes in xcb or it crashes because the connection is null.

At the same time we also ensure that the connection internally is
reset to null. Note: the one in kwinglobals.h caches and could cause
use-after-free errors. Any tear-down code must be migrated to
kwinApp()->x11Connection().

autotests/wayland/kwin_wayland_test.cpp

@@ -56,6 +56,7 @@ WaylandTestApplication::~WaylandTestApplication()
         Xcb::setInputFocus(XCB_INPUT_FOCUS_POINTER_ROOT);
         destroyAtoms();
         xcb_disconnect(x11Connection());
+        setX11Connection(nullptr);
     }
     if (m_xwaylandProcess) {
         m_xwaylandProcess->terminate();

composite.cpp

@@ -443,10 +443,12 @@ void Compositor::deleteUnusedSupportProperties()
         m_unusedSupportPropertyTimer.start();
         return;
     }
+    if (kwinApp()->x11Connection()) {
         foreach (const xcb_atom_t &atom, m_unusedSupportProperties) {
             // remove property from root window
             xcb_delete_property(connection(), rootWindow(), atom);
         }
+    }
 }
 
 // OpenGL self-check failed, fallback to XRender

main_wayland.cpp

@@ -80,6 +80,7 @@ ApplicationWayland::~ApplicationWayland()
         Xcb::setInputFocus(XCB_INPUT_FOCUS_POINTER_ROOT);
         destroyAtoms();
         xcb_disconnect(x11Connection());
+        setX11Connection(nullptr);
     }
     if (m_xwaylandProcess) {
         m_xwaylandProcess->terminate();