isXYObject = function(obj) {
            return obj instanceof $.Point || (typeof obj.x === 'number' && typeof obj.y === 'number');
        }
Sets an array of polygons to crop the TiledImage during draw tiles.
The render function will use the default non-zero winding rule.
@param {OpenSeadragon.Point[][]} polygons - represented in an array of point object in image coordinates.
Example format: [
 [{x: 197, y:172}, {x: 226, y:172}, {x: 226, y:198}, {x: 197, y:198}], // First polygon
 [{x: 328, y:200}, {x: 330, y:199}, {x: 332, y:201}, {x: 329, y:202}]  // Second polygon
 [{x: 321, y:201}, {x: 356, y:205}, {x: 341, y:250}] // Third polygon
]