EMI: Reset sort order when changing sets

Some sets (e.g. the big monkey's engine room) set Guybrush's sort order
to a large value, causing him to become invisible in other places. This
patch resets the sort order for all actors when switching to a new set.

Using this patch and Christian's actor collision implementation the game should be completable, just in time for Christmas. :-) I'll try to make a full run tomorrow.