Update polygon.class.js

Apply opacity property to poligon class.

src/shapes/polygon.class.js

@@ -123,6 +123,7 @@
     _render: function(ctx) {
       var point;
       ctx.beginPath();
+      ctx.globalAlpha = this.group ? (ctx.globalAlpha * this.opacity) : this.opacity;
       ctx.moveTo(this.points[0].x, this.points[0].y);
       for (var i = 0, len = this.points.length; i < len; i++) {
         point = this.points[i];