EMI: Return to original sort order when no sort plane is available

The floor in the Hall of Justice is hidden by a solid gray actor named
"trap plane". This actor is originally positioned far in the background
but moved to the foreground because we can't find a sort plane in the
sector.

This patch stores returns an actor to the original sort order set by
the game script when entering a sector that has no sort information.
Merge pull request #761 from inguin/emi-fixes

EMI: Fix invisible floor in the Hall of Justice