Do not use stackingOrder in LanczosFilter to get all EffectWindows

LanczosFilter tries to discard all cached textures in the timer event by
getting the stacking order and iterating over it. But this approach seems
wrong from several aspects.

First of all the xStackingOrder does not include Deleted windows. So if
a cached texture still exists on an EffectWindow for a Deleted it would
not be discarded.

Also the xStackingOrder could result in an update from X because the
stacking order is currently considered dirty.

Last but not least the EffectsHandler::stackingOrder creates a temporary
list of EffectWindows - good for Effects but not necessarily useful
inside KWin core.

Instead the LanczosFilter gets the list of Clients, desktops, unmanaged
and deleted and iterates over them to check whether there is a texture to
discard.

REVIEW: 109954