image/draw: add FloydSteinberg Drawer example

Updates #16360

Change-Id: I80b981aa291a8e16d2986d4a2dfd84d3819bf488
Reviewed-on: https://go-review.googlesource.com/29443
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/image/draw/example_test.go

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package draw_test
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+	"image/draw"
+	"math"
+)
+
+func ExampleDrawer_floydSteinberg() {
+	const width = 130
+	const height = 50
+
+	im := image.NewGray(image.Rectangle{Max: image.Point{X: width, Y: height}})
+	for x := 0; x < width; x++ {
+		for y := 0; y < height; y++ {
+			dist := math.Sqrt(math.Pow(float64(x-width/2), 2)/3+math.Pow(float64(y-height/2), 2)) / (height / 1.5) * 255
+			var gray uint8
+			if dist > 255 {
+				gray = 255
+			} else {
+				gray = uint8(dist)
+			}
+			im.SetGray(x, y, color.Gray{Y: 255 - gray})
+		}
+	}
+	pi := image.NewPaletted(im.Bounds(), []color.Color{
+		color.Gray{Y: 255},
+		color.Gray{Y: 160},
+		color.Gray{Y: 70},
+		color.Gray{Y: 35},
+		color.Gray{Y: 0},
+	})
+
+	draw.FloydSteinberg.Draw(pi, im.Bounds(), im, image.ZP)
+	shade := []string{" ", "░", "▒", "▓", "█"}
+	for i, p := range pi.Pix {
+		fmt.Print(shade[p])
+		if (i+1)%width == 0 {
+			fmt.Print("\n")
+		}
+	}
+}