clear m_activeEffects when effects change

If a screen with different refreshrate is attached, kwin currently resets
the compositor, altering the effect list.
If at the same time quads are rebuilt, the iteration operates on a pot.
invalid list of dangeling active effects.

Many thanks to Toralf Förster for his testing efforts.

BUG: 308201
FIXED-IN: 4.10.3
REVIEW: 110294
CC: release-team@kde.org