Destroy static Client helper window before application goes down

Summary:
Client uses a static Xcb::Window helper. This so far didn't get
explicitly destroyed, so the application finalize cleaned it up.
To destroy the window the xcb_connection_t* is used which the
QGuiApplication already destroyed.

This change ensures that the window gets destroyed before the xcb
connection gets destroyed.

In addition an assert is added to KWin::connection() to ensure that
we still have the QGuiApplication::instance() when it's invoked.
This way we'll notice if we have more cases where we call into xcb
after the application went down.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1573

client.cpp

@@ -630,6 +630,11 @@ void Client::updateShape()
 
 static Xcb::Window shape_helper_window(XCB_WINDOW_NONE);
 
+void Client::cleanupX11()
+{
+    shape_helper_window.reset();
+}
+
 void Client::updateInputShape()
 {
     if (hiddenPreview())   // Sets it to none, don't change

client.h

@@ -355,6 +355,8 @@ public:
      **/
     void showOnScreenEdge();
 
+    static void cleanupX11();
+
 public Q_SLOTS:
     void closeWindow() override;
     void updateCaption();

libkwineffects/kwinglobals.h

@@ -144,6 +144,7 @@ KWIN_EXPORT xcb_connection_t *connection()
     if (!s_con) {
         s_con = reinterpret_cast<xcb_connection_t*>(qApp->property("x11Connection").value<void*>());
     }
+    Q_ASSERT(qApp);
     return s_con;
 }
 

workspace.cpp

@@ -448,6 +448,7 @@ Workspace::~Workspace()
         m_allClients.removeAll(c);
         desktops.removeAll(c);
     }
+    Client::cleanupX11();
     for (UnmanagedList::iterator it = unmanaged.begin(), end = unmanaged.end(); it != end; ++it)
         (*it)->release(ReleaseReason::KWinShutsDown);
     xcb_delete_property(connection(), rootWindow(), atoms->kwin_running);