Add R_RelativeTeleportViewInterpolation

Adjusts pview_old for relative teleport to attempt a contigious motion

src/p_spec.c

@@ -2395,6 +2395,7 @@ static void P_ProcessLineSpecial(line_t *line, mobj_t *mo, sector_t *callsec)
 								camera[i].y += y;
 								camera[i].z += z;
 								camera[i].subsector = R_PointInSubsector(camera[i].x, camera[i].y);
+								R_RelativeTeleportViewInterpolation(i, x, y, z, 0);
 								break;
 							}
 						}

src/r_fps.c

@@ -210,6 +210,14 @@ void R_ResetViewInterpolation(UINT8 p)
 	}
 }
 
+void R_RelativeTeleportViewInterpolation(UINT8 p, fixed_t xdiff, fixed_t ydiff, fixed_t zdiff, angle_t angdiff)
+{
+	pview_old[p].x += xdiff;
+	pview_old[p].y += ydiff;
+	pview_old[p].z += zdiff;
+	pview_old[p].angle += angdiff;
+}
+
 void R_SetViewContext(enum viewcontext_e _viewcontext)
 {
 	UINT8 i = 0;

src/r_fps.h

@@ -115,6 +115,8 @@ void R_InterpolateView(fixed_t frac);
 void R_UpdateViewInterpolation(void);
 // Reset the view states (e.g. after level load) so R_InterpolateView doesn't interpolate invalid data
 void R_ResetViewInterpolation(UINT8 p);
+// Update old view for seamless relative teleport
+void R_RelativeTeleportViewInterpolation(UINT8 p, fixed_t xdiff, fixed_t ydiff, fixed_t zdiff, angle_t angdiff);
 // Set the current view context (the viewvars pointed to by newview)
 void R_SetViewContext(enum viewcontext_e _viewcontext);