Added zero point

pointf.go

@@ -10,6 +10,9 @@ type PointF struct {
 	X, Y float64
 }
 
+// ZeroPtF is initialized on (0, 0).
+var ZeroPtF = PointF{X: 0, Y: 0}
+
 // PtF is a shorthand function to create a point.
 func PtF(x, y float64) PointF {
 	return PointF{X: x, Y: y}