EMI: invalidate sortorder after detaching 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
- invalidate the sortorder also when detaching actors
- 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)
Merge pull request #804 from chkr-private/sortorder-fixes

EMI: invalidate sortorder after changing attached actors
Merge branch 'master' of https://github.com/residualvm/residualvm into subtitles