WarningMarker to uses Polygon instead of Rectangle

The google.maps.Rectangle object can sometimes be drawn strangly where it is stretched all across the
world horizontally.  This doesn't happen with the diamond shaped ErrorMarker.  Turns out that using
SVG-style paths with google.maps.Polygon doesn't have this problem, so the drawing of rectangles for
WarningMarker has been changed to use Polygon instead.