Create hardstop_gradients_many test slide.

This GM presents different gradients with an increasing number of
hardstops, from 1 to 100.

Change-Id: I1c279c6ea1a25f9785001aa29db632547a38ab68
Bug: skia:8401
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/457437
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

gm/hardstop_gradients_many.cpp

@@ -0,0 +1,90 @@
+/*
+ * Copyright 2021 Google LLC
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/*
+ * This GM presents different gradients with an increasing number of
+ * hardstops, from 1 to 100.
+ */
+
+#include "gm/gm.h"
+#include "include/core/SkCanvas.h"
+#include "include/core/SkColor.h"
+#include "include/core/SkPaint.h"
+#include "include/core/SkPoint.h"
+#include "include/core/SkRect.h"
+#include "include/core/SkRefCnt.h"
+#include "include/core/SkScalar.h"
+#include "include/core/SkShader.h"
+#include "include/core/SkSize.h"
+#include "include/core/SkString.h"
+#include "include/core/SkTileMode.h"
+#include "include/effects/SkGradientShader.h"
+
+const int kWidth  = 1000;
+const int kHeight = 2000;
+const int kNumRows = 100;
+const int kCellHeight = kHeight / kNumRows;
+const int kPadHeight = 1;
+const int kRectHeight = kCellHeight - (2 * kPadHeight);
+
+class HardstopGradientsManyGM : public skiagm::GM {
+public:
+    HardstopGradientsManyGM() {}
+
+protected:
+    SkString onShortName() override {
+        return SkString("hardstop_gradients_many");
+    }
+
+    SkISize onISize() override {
+        return SkISize::Make(kWidth, kHeight);
+    }
+
+    void onDraw(SkCanvas* canvas) override {
+        static constexpr SkPoint points[] = {
+            SkPoint::Make(0,      kRectHeight / 2),
+            SkPoint::Make(kWidth, kRectHeight / 2),
+        };
+
+        std::vector<SkColor> colors;
+        std::vector<SkScalar> positions;
+
+        for (int row = 1; row <= kNumRows; ++row) {
+            // Assemble a gradient containing a blue-to-white blend, repeated N times per row.
+            colors.push_back(SK_ColorBLUE);
+            colors.push_back(SK_ColorWHITE);
+
+            positions = {0.0f};
+            for (int pos = 1; pos < row; ++pos) {
+                float place = SkScalar(pos) / SkScalar(row);
+                positions.push_back(place);
+                positions.push_back(place);
+            }
+            positions.push_back(1.0f);
+            SkASSERT(positions.size() == colors.size());
+
+            // Draw it.
+            auto shader = SkGradientShader::MakeLinear(points,
+                                                       colors.data(),
+                                                       positions.data(),
+                                                       colors.size(),
+                                                       SkTileMode::kClamp,
+                                                       /*flags=*/0,
+                                                       /*localMatrix=*/nullptr);
+            SkPaint paint;
+            paint.setShader(shader);
+            canvas->drawRect(SkRect::MakeXYWH(0, kPadHeight, kWidth, kRectHeight), paint);
+
+            canvas->translate(0, kCellHeight);
+        }
+    }
+
+private:
+    using INHERITED = skiagm::GM;
+};
+
+DEF_GM(return new HardstopGradientsManyGM;)

gn/gm.gni

@@ -200,6 +200,7 @@ gm_sources = [
   "$_gm/hairlines.cpp",
   "$_gm/hairmodes.cpp",
   "$_gm/hardstop_gradients.cpp",
+  "$_gm/hardstop_gradients_many.cpp",
   "$_gm/highcontrastfilter.cpp",
   "$_gm/hittestpath.cpp",
   "$_gm/hsl.cpp",