Rework movie loading to fix loading built-in robot movies. The previous
code always ignored loading the last entry in the movie array, which was
blank until 18f6919b33. The last entry
was unused after d38a3d1362, so it was
removed, which caused the code to skip the robots entry.
Commit d38a3d1362 broke unloading add-on
movies by removing the strcpy which saved their name. Add code to save
the name to extra_robot_movie_mission and call it when switching
missions.
Reported by btb: https://github.com/dxx-rebirth/dxx-rebirth/pull/17
73b0a016a8