Do not allow rolling when riding.

Rolling while riding does not make sense, is not handled/animated at all,
and it looks strange. I wonder why nobody has fixed this in all this time.
Add Lua function isShuttingDownGameState()
Get rid of a few unnecessary continueity resets.

Continuity resets load a bunch of small files from disk;
this speeds up the startup precess very slightly if the files
are not in the OS's file cache yet.