[Reland] Fix SkTwoPointConicalGradient zero-radius handling

tests/GradientTest.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCanvas.h"
+#include "SkColorPriv.h"
 #include "SkColorShader.h"
 #include "SkGradientShader.h"
 #include "SkShader.h"
 #include "SkSurface.h"
 #include "SkTemplates.h"
 #include "Test.h"
 
 // https://code.google.com/p/chromium/issues/detail?id=448299
 // Giant (inverse) matrix causes overflow when converting/computing using 32.32
 // Before the fix, we would assert (and then crash).
@@ skipping 207 matching lines @@
 
     SkAutoTUnref<SkShader> gradient(
                    SkGradientShader::CreateLinear(pts, colors, pos, 4, SkShader::kClamp_TileMode));
 
     SkPaint paint;
     paint.setShader(gradient);
     SkRect r = {0, 83, 1254, 620};
     surface->getCanvas()->drawRect(r, paint);
 }
 
+// https://bugs.chromium.org/p/skia/issues/detail?id=5023
+// We should still shade pixels for which the radius is exactly 0.
+static void test_two_point_conical_zero_radius(skiatest::Reporter* reporter) {
+    SkAutoTUnref<SkSurface> surface(SkSurface::NewRasterN32Premul(5, 5));
+    surface->getCanvas()->clear(SK_ColorRED);
+
+    const SkColor colors[] = { SK_ColorGREEN, SK_ColorBLUE };
+    SkAutoTUnref<SkShader> shader(SkGradientShader::CreateTwoPointConical(
+        SkPoint::Make(2.5f, 2.5f), 0,
+        SkPoint::Make(3.0f, 3.0f), 10,
+        colors, nullptr, SK_ARRAY_COUNT(colors), SkShader::kClamp_TileMode));
+    SkPaint p;
+    p.setShader(shader);
+    surface->getCanvas()->drawPaint(p);
+
+    // r == 0 for the center pixel.
+    // verify that we draw it (no red bleed)
+    SkPMColor centerPMColor;
+    surface->readPixels(SkImageInfo::MakeN32Premul(1, 1), &centerPMColor, sizeof(SkPMColor), 2, 2);
+    REPORTER_ASSERT(reporter, SkGetPackedR32(centerPMColor) == 0);
+}
+
 DEF_TEST(Gradient, reporter) {
     TestGradientShaders(reporter);
     TestConstantGradient(reporter);
     test_big_grad(reporter);
     test_nearly_vertical(reporter);
     test_linear_fuzz(reporter);
+    test_two_point_conical_zero_radius(reporter);
 }