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/scripting/backend/codegen_doom.cpp

@@ -926,6 +926,11 @@ static DObject *BuiltinNewDoom(PClass *cls, int outerside, int backwardscompatib
 		ThrowAbortException(X_OTHER, "Cannot create actors with 'new'");
 		return nullptr;
 	}
+	if (cls->IsDescendantOf(NAME_ZSprite)) // Same for ZSprites.
+	{
+		ThrowAbortException(X_OTHER, "Cannot create ZSprite or inheriting classes with 'new'. Use 'ZSprite.Spawn' instead.");
+		return nullptr;
+	}
 	if ((vm_warnthinkercreation || !backwardscompatible) && cls->IsDescendantOf(NAME_Thinker))
 	{
 		// This must output a diagnostic warning