The image or texture that will repeated over a stroke.
Note that this image will be compressed into a square - so for best results have a square image. The rendered stamps will be the width of the line that the GMSStrokeStyle is set on. The image will be oriented with the top of the image towards the start point, and the bottom of the image towards the end point. For example, if the underlying line has two points and the start point is directly above the endpoint, the stamp will appear in an upright orientation.