EMI: invalidate sortorder after changing attached actors

- the order of the active actors is based on the effective sort order
  which is based on the sortorder of actors itself but also on the
  attached actors
- so the sortorder must also be invalidated when actors are attached or
  detached
- this fixes two glitches in the final end scene (guybrush is still
  displayed after falling from the cliff and monkey robot is drawn in
  the foreground)