add a GM to demo dither

Change-Id: Idbed35f2c5a1420b66b68725761a7ceea8cdd3ba
Reviewed-on: https://skia-review.googlesource.com/66461
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gm/dither.cpp

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2017 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+#include "SkSurface.h"
+#include "SkGradientShader.h"
+
+// This GM should make dithering somewhat visible when drawn into 565.
+
+DEF_SIMPLE_GM(dither, canvas, 256,256) {
+    // Create a compatible surface that's 8x8, as large as the largest dither pattern we use.
+    auto surface = canvas->makeSurface(canvas->imageInfo().makeWH(8,8));
+    if (!surface) {
+        surface = SkSurface::MakeRasterN32Premul(8,8);
+    }
+
+    // Draw a vertical red->green gradient.
+    SkPaint paint;
+    paint.setBlendMode(SkBlendMode::kSrc);
+    paint.setDither(true);
+    SkPoint    pts[] = { {0,0}, {0,8} };
+    SkColor colors[] = { 0xffff0000, 0xff00ff00 };
+    paint.setShader(SkGradientShader::MakeLinear(pts, colors, nullptr, 2,
+                                                 SkShader::kClamp_TileMode));
+    surface->getCanvas()->drawPaint(paint);
+
+    // Snap that and draw with a 32x zoom.
+    canvas->scale(32,32);
+    canvas->drawImage(surface->makeImageSnapshot(), 0,0);
+}

gn/gm.gni

@@ -102,6 +102,7 @@ gm_sources = [
   "$_gm/discard.cpp",
   "$_gm/displacement.cpp",
   "$_gm/distantclip.cpp",
+  "$_gm/dither.cpp",
   "$_gm/downsamplebitmap.cpp",
   "$_gm/draw_bitmap_rect_skbug4374.cpp",
   "$_gm/drawable.cpp",