Added A_SetViewPos(Vector3 Offset, int Flags = -1)

- Offset: The offset from the actor's view to move the camera about.
 - Flags: (Default is -1, which means don't change flags)
  - VPSF_ABSOLUTEOFFSET: Don't include actor angles in calculation.
  - VPSF_ABSOLUTEPOS: Position is absolute, and disables all transformations. Modders are responsible for being portal aware!

Notes:
- `ViewPos` in Actor will be `null` until A_SetViewPos is called for the first time.

**Issues:**
- Hiding sprite while in portal incomplete.

src/playsim/p_mobj.cpp

@@ -369,6 +369,7 @@ void AActor::Serialize(FSerializer &arc)
 		A("spawnorder", SpawnOrder)
 		A("friction", Friction)
 		A("SpriteOffset", SpriteOffset)
+		("viewpos", ViewPos)
 		A("userlights", UserLights);
 
 		SerializeTerrain(arc, "floorterrain", floorterrain, &def->floorterrain);
@@ -4952,6 +4953,12 @@ void AActor::OnDestroy ()
 	UnlinkFromWorld (nullptr);
 	flags |= MF_NOSECTOR|MF_NOBLOCKMAP;
 
+	if (ViewPos != nullptr)
+	{
+		ViewPos->Destroy();
+		ViewPos = nullptr;
+	}
+
 	// Transform any playing sound into positioned, non-actor sounds.
 	S_RelinkSound (this, NULL);