scene: Improve window quad management

At the moment, we handle window quads inefficiently. Window quads from
all items are merged into a single list just to be broken up again.

This change removes window quads from libkwineffects. This allows us to
handle window quads efficiently. Furthermore, we could optimize methods
such as WindowVertex::left() and so on. KWin spends reasonable amount
of time in those methods when many windows have to be composited.

It's a necessary prerequisite for making wl_surface painting code role
agnostic.