Support shutting down the render thread without an OpenGL context.

[ChangeLog][QtQuick][SceneGraph] There might not be an OpenGL context
bound when QQuickWindow::sceneGraphInvalidated() is emitted if an
error occurs while cleaning up the scene graph (such as
EGL_CONTEXT_LOST). This is according to the documentation, but has
never occurred in practice before.

Change-Id: I13dbfbb4b6d0d27fa42fcb8b54df16ea02284807
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>