effects/thumbnailaside: Fix crash in clientArea()
clientArea() was changed to forbid "-1" screen ids, but it seems like the thumbnail aside effect can pass it to refer to the active screen.
This change makes the thumbnail aside effect handle "-1" screen id case explicitly. It will be also useful for EffectScreen transition.
Allow passing -1 screen id to clientArea() until the effects are ported to EffectScreen.