Add WorldPanning flag for camera texture definitions.

- To make camera textures pan in world units instead of texture units, you
  can now add "WorldPanning" at the end of the cameratexture definition in
  ANIMDEFS, after the "fit" specification. e.g.

    cameratexture CAMTEX 128 128 fit 64 64 worldpanning

src/textures/animations.cpp

@@ -659,6 +659,17 @@ void FTextureManager::ParseCameraTexture(FScanner &sc)
 			sc.UnGet ();
 		}
 	}
+	if (sc.GetString())
+	{
+		if (sc.Compare("WorldPanning"))
+		{
+			viewer->bWorldPanning = true;
+		}
+		else
+		{
+			sc.UnGet();
+		}
+	}
 	viewer->SetScaledSize(fitwidth, fitheight);
 }