QtCreator allocates many pixmaps on X....

According to xrestop on X11, QtCreator allocates many XPixmaps on X. I don't know if it's a bug in Qt but others apps don't have so many XPixmaps allocated in X.
A session with let say gui.pro of Qt opened shows 4201 pixmaps. As an example plasma-desktop allocates 470 pixmaps on X (with x11 paint engine), chromium 147. After some times X becomes really slow and killing QtCreator brings it back to normal speed.

With graphicssystem raster it's way better even though changing sessions seems to leak pixmaps on X (2-3 pixmaps).