Port xRenderOffscreenTarget away from QPixmap
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.
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