Port Screenshot effect to XCB

This includes:
* getting cursor image through xcb_xfixes
* using xcb_image_get for reading screen content in xrender case
* using xcb_put_image to upload the generated screenshot into shared xpix

Overall this means that QPixmap as an X Pixmap wrapper is no longer used.
The conversion from xcb_image_t to QImage still needs some code for not
matching byte order. I'm a little bit unsure about adding the code as I
would not be able to test it.

REVIEW: 109076