Use Analytic AA in SkAAClip

src/core/SkAnalyticEdge.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkAnalyticEdge_DEFINED
 #define SkAnalyticEdge_DEFINED
 
@@ skipping 25 matching lines @@
                             // fDY is only used for blitting trapezoids.
 
     int8_t  fCurveCount;    // only used by kQuad(+) and kCubic(-)
     uint8_t fCurveShift;    // appled to all Dx/DDx/DDDx except for fCubicDShift exception
     uint8_t fCubicDShift;   // applied to fCDx and fCDy only in cubic
     int8_t  fWinding;       // 1 or -1
 
     static const int kDefaultAccuracy = 2; // default accuracy for snapping
 
     static inline SkFixed snapY(SkFixed y, int accuracy = kDefaultAccuracy) {
-        return SkFixedRoundToFixed(y << accuracy) >> accuracy;
+        // This approach is safer than left shift, round, then right shift
+        return (y + (SK_Fixed1 >> (accuracy + 1))) >> (16 - accuracy) << (16 - accuracy);
     }
 
     // Update fX, fY of this edge so fY = y
     inline void goY(SkFixed y) {
         if (y == fY + SK_Fixed1) {
             fX = fX + fDX;
             fY = y;
         } else if (y != fY) {
             // Drop lower digits as our alpha only has 8 bits
             // (fDX and y - fUpperY may be greater than SK_Fixed1)
@@ skipping 88 matching lines @@
     fWinding    = SkToS8(winding);
     fCurveShift = 0;
 
     if (clip) {
         this->chopLineWithClip(*clip);
     }
     return true;
 }
 
 #endif