Keep QPixmap copy of screen contents using X11, XDamage, XRender, and other tricks

I'm attempting to solve what I thought would be a very simple problem. I want to keep a QPixmap updated with the entire screen contents. You can get such a pixmap by doing this:

QDesktopWidget *w = QApplication::desktop();
if (w)
{
    QRect r = w->screenGeometry();
    QPixmap p = QPixmap::grabWindow(w->winId(), 0, 0, r.width(), r.height())
    QByteArray bitmap;
}

The problem with this is that QDesktopWidget ends up re-grabbing the entire screen pixmap from the X11 server every time you ask for it, even if nothing has changed.

I need this code to be fast, so I'm trying to do this myself. My starting point was the qx11mirror demo, however, that basically does the same thing. It uses the XDamage extension to work out when something has changed, but instead of using the damaged rectangle information to just update that part of the cached pixmap, it just sets a "dirty" flag, which triggers an entire refresh anyway.

So I'm trying to modify the qx11mirror example to just update the damaged portion of the windows, but I can't seem to get anything to work - all I get is a blank (black) pixmap. The code I'm using is:

void QX11Mirror::x11Event(XEvent *event)
{
    if (event->type == m_damageEvent + XDamageNotify)
    {
        XDamageNotifyEvent *e = reinterpret_cast<XDamageNotifyEvent*>(event);

        XWindowAttributes attr;
        XGetWindowAttributes(QX11Info::display(), m_window, &attr);
        XRenderPictFormat *format = XRenderFindVisualFormat(QX11Info::display(), attr.visual);
        bool hasAlpha             = ( format->type == PictTypeDirect && format->direct.alphaMask );
        int x                     = attr.x;
        int y                     = attr.y;
        int width                 = attr.width;
        int height                = attr.height;

            // d