effects/desktopgrid: Clip windows differently

Similar to the slide effect, the desktop grid can use the clip region to
clip windows when needed. This will improve performance because the desktop
grid effect will do less work. Currently, it clips quads on its own in
prePaintWindow, copies them over in a temporary list, and lets the opengl
scene clip the window quads again.

We can clip windows differently by just passing a custom region to the
paintWindow() function down the effects chain, which the desktop grid
effect already does.

The old behavior and the old bugs are preserved. The Xrender code path is
unaffected.