provide patch accepted by reviewboard ...

As proposed this allows to invert the screen with zero overhead or requirement of OpenGL compositing.
The patch depends on https://git.reviewboard.kde.org/r/103058/ which adds the libXxf86vm dependency (until i link out that nvidia refreshrate detection part)

Notice that it inverts the display colors - one cannot make an inverted screenshot this way.

Inverts the screen.
However there's an issue with the screen parameter. Neither active_screen nor QX11Info().screen() does work (unless on the primary screen, "0") - at least not on a TwinView setup.
We have to try xrandr and xinerama, but it will linkey only work on independent screens or even X11 servers *shrug*