Move MT_RAY to the very end of the mobj list

!1729 (merged) added MT_RAY, a general purpose mobj meant to replace any usage of MT_NULL, which was added right after MT_NULL in the mobj list, at the very start. This shifted over every mobj number by 1, which sadly causes issues when playing back any demo with a character that has a followitem, as those are written as numbers into the demo format.

This branch moves MT_RAY to the very end of the list. While it could be possible to make demos account for the changed mobj numbers, this seems like a much easier solution.