Fix quad convexity with only one backward dir

tests/PathTest.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 <cmath>
 #include "SkCanvas.h"
 #include "SkGeometry.h"
@@ skipping 1478 matching lines @@
     path.reset();
     path.addRect(0, 0, SkIntToScalar(10), SkIntToScalar(10), SkPath::kCCW_Direction);
     check_convexity(reporter, path, SkPath::kConvex_Convexity);
     REPORTER_ASSERT(reporter, SkPathPriv::CheapIsFirstDirection(path, SkPathPriv::kCCW_FirstDirection));
 
     path.reset();
     path.addRect(0, 0, SkIntToScalar(10), SkIntToScalar(10), SkPath::kCW_Direction);
     check_convexity(reporter, path, SkPath::kConvex_Convexity);
     REPORTER_ASSERT(reporter, SkPathPriv::CheapIsFirstDirection(path, SkPathPriv::kCW_FirstDirection));
 
+    path.reset();
+    path.quadTo(100, 100, 50, 50); // This is a convex path from GM:convexpaths
+    check_convexity(reporter, path, SkPath::kConvex_Convexity);
+
     static const struct {
         const char*                 fPathStr;
         SkPath::Convexity           fExpectedConvexity;
         SkPathPriv::FirstDirection  fExpectedDirection;
     } gRec[] = {
         { "", SkPath::kConvex_Convexity, SkPathPriv::kUnknown_FirstDirection },
         { "0 0", SkPath::kConvex_Convexity, SkPathPriv::kUnknown_FirstDirection },
         { "0 0 10 10", SkPath::kConvex_Convexity, SkPathPriv::kUnknown_FirstDirection },
         { "0 0 10 10 20 20 0 0 10 10", SkPath::kConcave_Convexity, SkPathPriv::kUnknown_FirstDirection },
         { "0 0 10 10 10 20", SkPath::kConvex_Convexity, SkPathPriv::kCW_FirstDirection },
@@ skipping 2933 matching lines @@
     PathRefTest_Private::TestPathRef(reporter);
     PathTest_Private::TestPathrefListeners(reporter);
     test_dump(reporter);
     test_path_crbug389050(reporter);
     test_path_crbugskia2820(reporter);
     test_skbug_3469(reporter);
     test_skbug_3239(reporter);
     test_bounds_crbug_513799(reporter);
     test_fuzz_crbug_638223();
 }