effectshandler: Reset iterators when active effects changes

Clearing and resizing m_activeEffects invalidates iterators, so we need
to ensure we reset any iterators that were stored.

src/effects.cpp

@@ -1574,6 +1574,10 @@ void EffectsHandlerImpl::effectsChanged()
               std::back_inserter(loaded_effects));
 
     m_activeEffects.reserve(loaded_effects.count());
+
+    m_currentPaintScreenIterator = m_activeEffects.constBegin();
+    m_currentPaintWindowIterator = m_activeEffects.constBegin();
+    m_currentDrawWindowIterator = m_activeEffects.constBegin();
 }
 
 QStringList EffectsHandlerImpl::activeEffects() const