wasm: remove window from requestUpdate queue on removeWindow()

Don't deliver update requests to windows which have been deleted
or are not on screen.

Change-Id: Ia2972e8dbef46eaf91a45a84962353917d436da6
Reviewed-by: Aleksandr Reviakin <aleksandr.reviakin@qt.io>
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>
Reviewed-by: David Skoland <david.skoland@qt.io>

src/plugins/platforms/wasm/qwasmcompositor.cpp

@@ -238,6 +238,7 @@ void QWasmCompositor::removeWindow(QWasmWindow *window)
 
     m_windowStack.removeAll(window);
     m_compositedWindows.remove(window);
+    m_requestUpdateWindows.remove(window);
 
     if (!m_windowStack.isEmpty() && !QGuiApplication::focusWindow()) {
         auto lastWindow = m_windowStack.last();