Revert "Don't invalidate textures when the scene graph is stopped."

This was too aggressive, if there is a queued buffer the current one
needs to be invalidated or the compositor will hold references to
both buffers while the scene graph is stopped which will block
clients in eglSwapBuffers.

This reverts commit 438063af

.

Change-Id: I3247cfc94e12322fcccc07b5dd64146b055ab431
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>

src/compositor/compositor_api/qwaylandquicksurface.cpp

@@ -208,6 +208,8 @@ bool QWaylandQuickSurface::event(QEvent *e)
                        this, &QWaylandQuickSurface::updateTexture);
             disconnect(oldWindow, &QQuickWindow::sceneGraphInvalidated,
                        this, &QWaylandQuickSurface::invalidateTexture);
+            disconnect(oldWindow, &QQuickWindow::sceneGraphAboutToStop,
+                       this, &QWaylandQuickSurface::invalidateTexture);
         }
 
         return true;
@@ -224,6 +226,9 @@ bool QWaylandQuickSurface::event(QEvent *e)
             connect(window, &QQuickWindow::sceneGraphInvalidated,
                     this, &QWaylandQuickSurface::invalidateTexture,
                     Qt::DirectConnection);
+            connect(window, &QQuickWindow::sceneGraphAboutToStop,
+                    this, &QWaylandQuickSurface::invalidateTexture,
+                    Qt::DirectConnection);
         }
 
         return true;