fix zero-length tangent

gm/strokes.cpp

Index: gm/strokes.cpp
diff --git a/gm/strokes.cpp b/gm/strokes.cpp
index 73823907a37c1d613e55ca77221cf7cab2188b5d..cfc88f343d0a9102b0bdc4089ae587a4ae3fae8a 100644
--- a/gm/strokes.cpp
+++ b/gm/strokes.cpp
@@ -266,6 +266,60 @@ private:
     typedef skiagm::GM INHERITED;
 };
 
+// Test stroking for curves that produce degenerate tangents when t is 0 or 1 (see bug 4191)
+class Strokes5GM : public skiagm::GM {
+public:
+    Strokes5GM() {}
+
+protected:
+
+    SkString onShortName() override {
+        return SkString("zero_control_stroke");
+    }
+
+    SkISize onISize() override {
+        return SkISize::Make(W, H*2);
+    }
+
+    void onDraw(SkCanvas* canvas) override {
+        SkPaint p;
+        p.setColor(SK_ColorRED);
+        p.setAntiAlias(true);
+        p.setStyle(SkPaint::kStroke_Style);
+        p.setStrokeWidth(40);
+        p.setStrokeCap(SkPaint::kButt_Cap);
+
+        SkPath path;
+        path.moveTo(157.474f,111.753f);
+        path.cubicTo(128.5f,111.5f,35.5f,29.5f,35.5f,29.5f);
+        canvas->drawPath(path, p);
+        path.reset();
+        path.moveTo(250, 50);
+        path.quadTo(280, 80, 280, 80);
+        canvas->drawPath(path, p);
+        path.reset();
+        path.moveTo(150, 50);
+        path.conicTo(180, 80, 180, 80, 0.707f);
+        canvas->drawPath(path, p);
+
+        path.reset();
+        path.moveTo(157.474f,311.753f);
+        path.cubicTo(157.474f,311.753f,85.5f,229.5f,35.5f,229.5f);
+        canvas->drawPath(path, p);
+        path.reset();
+        path.moveTo(280, 250);
+        path.quadTo(280, 250, 310, 280);
+        canvas->drawPath(path, p);
+        path.reset();
+        path.moveTo(180, 250);
+        path.conicTo(180, 250, 210, 280, 0.707f);
+        canvas->drawPath(path, p);
+    }
+
+private:
+    typedef skiagm::GM INHERITED;
+};
+
 
 //////////////////////////////////////////////////////////////////////////////
 
@@ -273,8 +327,10 @@ static skiagm::GM* F0(void*) { return new StrokesGM; }
 static skiagm::GM* F1(void*) { return new Strokes2GM; }
 static skiagm::GM* F2(void*) { return new Strokes3GM; }
 static skiagm::GM* F3(void*) { return new Strokes4GM; }
+static skiagm::GM* F4(void*) { return new Strokes5GM; }
 
 static skiagm::GMRegistry R0(F0);
 static skiagm::GMRegistry R1(F1);
 static skiagm::GMRegistry R2(F2);
 static skiagm::GMRegistry R3(F3);
+static skiagm::GMRegistry R4(F4);