some redesign

scale.go

@@ -38,7 +38,7 @@ func (r rescaled) At(x, y int) color.Color {
 // Scale scales image.Image to a given scale
 // and then returns image.Image.
 // For now it will work only with positive integers.
-func Scale(img image.Image, scale int) image.Image {
+func Scale(scale int, img image.Image) image.Image {
 	if scale < 1 {
 		scale = 1
 	}