clamp to premul in dither

Dither can bump color values above alpha (duh), or below
zero (duh), so clamp back to premul after dithering.

BUG=skia:6644,skia:6643

Change-Id: Ida107e866380e06130af0d01467117bca929ba44
Reviewed-on: https://skia-review.googlesource.com/17070
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gm/bug6643.cpp

@@ -0,0 +1,29 @@
+/*
+ * 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 "SkGradientShader.h"
+#include "SkPictureRecorder.h"
+#include "gm.h"
+
+DEF_SIMPLE_GM(bug6643, canvas, 200, 200) {
+    SkColor colors[] = { SK_ColorTRANSPARENT, SK_ColorGREEN, SK_ColorTRANSPARENT };
+
+    SkPaint p;
+    p.setAntiAlias(true);
+    p.setShader(SkGradientShader::MakeSweep(100, 100, colors, nullptr, SK_ARRAY_COUNT(colors),
+                                            SkGradientShader::kInterpolateColorsInPremul_Flag,
+                                            nullptr));
+
+    SkPictureRecorder recorder;
+    recorder.beginRecording(200, 200)->drawPaint(p);
+
+    p.setShader(SkShader::MakePictureShader(recorder.finishRecordingAsPicture(),
+                                            SkShader::kRepeat_TileMode, SkShader::kRepeat_TileMode,
+                                            nullptr, nullptr));
+    canvas->drawColor(SK_ColorWHITE);
+    canvas->drawPaint(p);
+}

gn/gm.gni

@@ -49,6 +49,7 @@ gm_sources = [
   "$_gm/blurs.cpp",
   "$_gm/bmpfilterqualityrepeat.cpp",
   "$_gm/bug5252.cpp",
+  "$_gm/bug6643.cpp",
   "$_gm/bug530095.cpp",
   "$_gm/bug615686.cpp",
   "$_gm/cgm.c",

src/jumper/SkJumper_stages.cpp

@@ -320,6 +320,10 @@ STAGE(dither) {
     r += c->rate*dither;
     g += c->rate*dither;
     b += c->rate*dither;
+
+    r = max(0, min(r, a));
+    g = max(0, min(g, a));
+    b = max(0, min(b, a));
 }
 
 // load 4 floats from memory, and splat them into r,g,b,a