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/rendering/hwrenderer/scene/hw_sprites.cpp

@@ -734,6 +734,12 @@ void HWSprite::Process(HWDrawInfo *di, AActor* thing, sector_t * sector, area_t
 		DVector3 thingorigin = thing->Pos();
 		if (thruportal == 1) thingorigin += di->Level->Displacements.getOffset(thing->Sector->PortalGroup, sector->PortalGroup);
 		if (fabs(thingorigin.X - vp.ActorPos.X) < 2 && fabs(thingorigin.Y - vp.ActorPos.Y) < 2) return;
+
+		// Try to do an inverted trace to get to the camera's body. This way, we know exactly behind which line to disable.
+		// Currently doesn't work.
+		FViewPosition *vpos = camera->ViewPos;
+		if (vpos && vpos->TraceBack(camera))
+			return;
 	}
 	// Thing is invisible if close to the camera.
 	if (thing->renderflags & RF_MAYBEINVISIBLE)