Fix flickering in slide effect with multi screen

The slide effect translates all windows as we switch desktop. In a multi
screen setup we don't want windows to enter a different screen during
this animation.

The current code masks everything to the current output. This is broken
for any window that spans across screens even slightly. It will not be
visible during the transform then appear on top at the end.

The new algorithm is, for each screen in each window we crop the painted
area to the intersection of the screen and the screen's translated
position.