Instead of having a pointer to a QPixmap the offscreen target holds an
xcb_render_picture_t. To make this possible in SceneWindow the tempPixmap
is changed from a QPixmap* to a XRenderPicture*. QPixmap was only used
for convenience.

ScreenShot Effect as only user of the offscreen target is adjusted but
as it needs a QImage, still uses a QPixmap wrapper.