Do not add QOffscreenSurface windows to the global list

QOffscreenSurface has to stay usable even after returning from app.exec().
Hence close()ing the underlying hidden window, that is used on platforms that
do not provide real offscreen surfaces, is wrong.

Normally all QWindows are closed (and thus destroy()'ed) when quitting the application,
meaning the the offscreen surface cannot be made current anymore after returning
from exec(). This is an unnecessary limitation and makes certain cleanup operations
impossible.

Task-number: QTBUG-39908
Change-Id: Iea1489378a18f29ff84ba8f13a6dad2d66d2b315
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>

src/gui/kernel/qoffscreensurface.cpp

@@ -171,6 +171,9 @@ void QOffscreenSurface::create()
             if (QThread::currentThread() != qGuiApp->thread())
                 qWarning("Attempting to create QWindow-based QOffscreenSurface outside the gui thread. Expect failures.");
             d->offscreenWindow = new QWindow(d->screen);
+            // Remove this window from the global list since we do not want it to be destroyed when closing the app.
+            // The QOffscreenSurface has to be usable even after exiting the event loop.
+            QGuiApplicationPrivate::window_list.removeOne(d->offscreenWindow);
             d->offscreenWindow->setSurfaceType(QWindow::OpenGLSurface);
             d->offscreenWindow->setFormat(d->requestedFormat);
             d->offscreenWindow->setGeometry(0, 0, d->size.width(), d->size.height());