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/playsim/dthinker.cpp

@@ -807,7 +807,12 @@ DEFINE_ACTION_FUNCTION_NATIVE(DThinker, ChangeStatNum, ChangeStatNum)
 {
 	PARAM_SELF_PROLOGUE(DThinker);
 	PARAM_INT(stat);
-	ChangeStatNum(self, stat);
+
+	// do not allow ZScript to reposition thinkers in or out of particle ticking.
+	if (stat != STAT_SPRITE && !dynamic_cast<DZSprite*>(self))
+	{
+		ChangeStatNum(self, stat);
+	}
 	return 0;
 }