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_map.cpp

@@ -5423,6 +5423,62 @@ void P_AimCamera(AActor *t1, DVector3 &campos, DAngle &camangle, sector_t *&Came
 	camangle = trace.SrcAngleFromTarget - 180.;
 }
 
+// [MC] Used for ViewPos. Uses code borrowed from P_AimCamera.
+void P_AdjustViewPos(AActor *t1, DVector3 orig, DVector3 &campos, sector_t *&CameraSector, bool &unlinked, FViewPosition *VP)
+{
+	FTraceResults trace;
+	const DVector3 vvec = campos - orig;
+	const double distance = vvec.Length();
+
+	// Trace handles all of the portal crossing, which is why there is no usage of Vec#Offset(Z).
+	if (Trace(orig, t1->Sector, vvec.Unit(), distance, 0, 0, t1, trace) &&
+		trace.Distance > 5)
+	{
+		// Position camera slightly in front of hit thing
+		campos = orig + vvec.Unit() * (trace.Distance - 5);
+	}
+	else
+	{
+		campos = trace.HitPos - trace.HitVector * 1 / 256.;
+	}
+//	DVector3 cpos = campos;
+	CameraSector = trace.Sector;
+	unlinked = trace.unlinked;
+
+	// Save the info for the renderers. Needed to disable sprites across portals.
+	VP->Distance = distance;
+	VP->isUsed = true;
+}
+
+struct EViewPosFinder
+{
+	AActor *Owner;
+};
+
+static ETraceStatus TraceToOwner(FTraceResults &res, void *userdata)
+{
+	// Try to guarantee hitting the owner if possible.
+	if (res.HitType != TRACE_HitActor)
+		return TRACE_Skip;
+
+	EViewPosFinder *data = (EViewPosFinder *)userdata;
+	ETraceStatus ret = (res.Actor == data->Owner) ? TRACE_Stop : TRACE_Skip;
+	return ret;
+}
+
+bool FViewPosition::TraceBack(AActor *Owner)
+{
+	if (!isUsed || !Owner)
+		return false;
+	
+	DVector3 vvec = (CamPos - PlrPos).Unit() * -1.0;
+	FTraceResults res;
+	EViewPosFinder TrcBack = {Owner};
+
+	bool ret = Trace(CamPos, CamSec, vvec, Distance, 0, 0, nullptr, res, TRACE_ReportPortals, TraceToOwner, &TrcBack);
+	
+	return ret;
+}
 
 //==========================================================================
 //