effects: Wire EffectsHandler::screenGeometryChanged() to Screens::geometryChanged()

Currently, the EffectsHandler has two signals that are emitted when the
combined geometry of all outputs change - virtualScreenGeometryChanged()
and screenGeometryChanged(). Having two signals is most likely a
historical artifact.

This change untangles the screenGeometryChanged() signal from the
Workspace and makes it the same as the virtualScreenGeometryChanged()
signal.