Added ZSprites.

- Effectively similar to Actors, but without the excess.
- Can be created with either the `level` function or the static `Spawn` function in ZSprite.
- STAT_SPRITE belongs to ZSprites only; ZSprites cannot be moved out of, nor can anything else be moved in to, this statnum.

Misc:

- Fixed (Sprite)Offset taking roll into account.

Crediting phantombeta, RicardoLuis0 and RaveYard for assistance.

src/common/engine/namedef.h

@@ -10,6 +10,7 @@ xx(Object)
 xx(Actor)
 xx(Class)
 xx(Thinker)
+xx(ZSprite)
 xx(Crosshairs)
 
 xx(Untranslated)