a paint device is recreated on every update

While implementing cairo support for Qt Quick applications, I've noticed that a paint device is always recreated for a QPaintedItem, even when the dimension of the item haven't changed. In other words, a QImage, with associated storage, which may be considerable, is recreated at every update. In code:

https://github.com/user1095108/cairowidget/blob/master/CairoPaintedItem.cpp

void CairoPaintedItem::paint(QPainter* const p)
{
auto cr(cr_);

auto const w(width()), h(height());

{
auto const img(static_cast<QImage*>(p->device()));

if (auto const d(img->bits()); (d != d_) || (w != w_) || (h != h_))
{
d_ = d; w_ = w; h_ = h;
//...

If you add qDebug() calls you will see that d is pretty much different every time update() on the item is called. QImage instances could be reused, making rendering faster.

Also see:
https://github.com/user1095108/cairowidget/blob/master/example7.cpp