This is *theoretically* possible for xrender compositing just as for
OpenGL w/o shaders (ie. not correctly preserving alpha) through
PictOpDifference, BUT i've tried and it only works on the nvidia blob
(intel just ignores the call) and is DOOOOOOOG SLOOOOOOW ;-)
For the entire screen it can however be done through
XF86VidModeSetGammaRamp w/o any overhead.
This approach however actually doesn't require a compositor at all.
Since we're about to conditionally link xvidmode in 4.9 anyway, this is
somehow about accessibility and there's talk to change the effect config
UI, i wonder whether we should offer this as shortcut from the core (if
xvidmode is present) and invert the screen regardless of the compositor
(about 5-10 LOC)