Reduce viewport invalidation area of signal state changes

src/signal.cpp

@@ -583,7 +583,7 @@ static void UpdateSignalsAroundSegment(SigInfo info)
 				MarkDependencidesForUpdate(SignalReference(tile, track));
 			}
 			SetSignalStateByTrackdir(tile, trackdir, newstate);
-			MarkTileDirtyByTile(tile, ZOOM_LVL_DRAW_MAP);
+			MarkSingleSignalDirty(tile, trackdir);
 		}
 	}