Fixing incorrect clipping of sliding popups

Make use of new extension of protocol for magic number -1.
If offset is -1 KWin has to decide the offset. This fixes all the
incorrect animations and allows us to perform clipping again by
filtering out the window quads which should not be visible.

Additionally the effect now sanitizes the offset. That is for e.g.
Yakuake setting an offset of 0, but there is a strut on the top
corner causing Yakuake not to appear on 0, but with an offset of
the strut. Such cases are now considered as well and the animation
is fixed.

REVIEW: 103367



FIXED-IN: 4.8.0