(svn r26900) -Fix-ish: dirty the appropriate area around map edges when terraforming there to prevent any artefacts from occuring (ic111)

src/viewport_func.h

@@ -80,6 +80,7 @@ extern Point _tile_fract_coords;
 void MarkTileDirtyByTile(TileIndex tile);
 
 int GetRowAtTile(int viewport_y, Point tile, bool bridge_correct);
+void MarkTileDirtyByTileOutsideMap(int x, int y);
 
 Point GetViewportStationMiddle(const ViewPort *vp, const Station *st);