Draw Alpha8 + color filter correctly in software

This makes the behavior match our gpu backend.

BUG=android:31019366

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3979

Change-Id: I941623d0bdf54f61aafb1383de46bbedceb7578d
Reviewed-on: https://skia-review.googlesource.com/3979
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>

gm/colorfilteralpha8.cpp

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2016 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 "SkBitmap.h"
+#include "SkColorFilter.h"
+
+class ColorFilterAlpha8 : public skiagm::GM {
+public:
+    ColorFilterAlpha8() {}
+
+protected:
+    SkString onShortName() override {
+        return SkString("colorfilteralpha8");
+    }
+
+    SkISize onISize() override {
+        return SkISize::Make(400, 400);
+    }
+
+    void onDraw(SkCanvas* canvas) override {
+        canvas->clear(SK_ColorRED);
+
+        SkBitmap bitmap;
+        SkImageInfo info = SkImageInfo::MakeA8(200, 200);
+        bitmap.allocPixels(info);
+        bitmap.eraseColor(0x88FFFFFF);
+
+        SkPaint paint;
+        float opaqueGrayMatrix[20] = {
+                0.0f, 0.0f, 0.0f, 1.0f, 0.0f,
+                0.0f, 0.0f, 0.0f, 1.0f, 0.0f,
+                0.0f, 0.0f, 0.0f, 1.0f, 0.0f,
+                0.0f, 0.0f, 0.0f, 0.0f, 255.0f
+        };
+        paint.setColorFilter(SkColorFilter::MakeMatrixFilterRowMajor255(opaqueGrayMatrix));
+
+        canvas->drawBitmap(bitmap, 100.0f, 100.0f, &paint);
+    }
+
+private:
+    typedef skiagm::GM INHERITED;
+};
+DEF_GM( return new ColorFilterAlpha8; )

gn/gm.gni

@@ -59,6 +59,7 @@ gm_sources = [
   "$_gm/color4f.cpp",
   "$_gm/colorcube.cpp",
   "$_gm/coloremoji.cpp",
+  "$_gm/colorfilteralpha8.cpp",
   "$_gm/colorfilterimagefilter.cpp",
   "$_gm/colorfilters.cpp",
   "$_gm/colormatrix.cpp",

src/core/SkDraw.cpp

@@ -1342,7 +1342,8 @@ void SkDraw::drawBitmap(const SkBitmap& bitmap, const SkMatrix& prematrix,
     SkDraw draw(*this);
     draw.fMatrix = &matrix;
 
-    if (bitmap.colorType() == kAlpha_8_SkColorType) {
+    if (bitmap.colorType() == kAlpha_8_SkColorType && !paint->getColorFilter() &&
+            !paint->getShader()) {
         draw.drawBitmapAsMask(bitmap, *paint);
     } else {
         SkAutoBitmapShaderInstall install(bitmap, *paint);