EMI: Use parent sort order for attached actors

When drawing attached actors the engine uses the parent's sort order
plus one. This offset can move the attached actor behind a background
plane, for instance the chess board in the Lucre docks or Guybrush when
riding the manatee.

The +1 offset was introduced in 58b0bed. The comment suggests that there
were some problems without it, but I don't see any. The z-buffer should take
care of which actor covers which.