Turn dispoffset into a mobj field

mobj->info->dispoffset is now just the default value for mobj->dispoffset, which is now the value referenced during vissprite creation. This means you can sort sprites per-mobj, which is infinitely more useful and resourceful than sorting by mobj type.

Example script: shielddispoffset.lua
Hold toss flag to invert the dispoffset of your shield, which is modified on the mobj's side rather than in mobjinfo.