Fix the issue of alpha = 254 instead of 255

src/core/SkAntiRun.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 SkAntiRun_DEFINED
 #define SkAntiRun_DEFINED
 
 #include "SkBlitter.h"
 
 /** Sparse array of run-length-encoded alpha (supersampling coverage) values.
     Sparseness allows us to independently compose several paths into the
     same SkAlphaRuns buffer.
 */
 
 class SkAlphaRuns {
 public:
     int16_t*    fRuns;
     uint8_t*     fAlpha;
 
+    // Return 0-255 given 0-256
+    static inline SkAlpha CatchOverflow(int alpha) {
+        return alpha - (alpha >> 8);
+    }
+
     /// Returns true if the scanline contains only a single run,
     /// of alpha value 0.
     bool empty() const {
         SkASSERT(fRuns[0] > 0);
         return fAlpha[0] == 0 && fRuns[fRuns[0]] == 0;
     }
 
     /// Reinitialize for a new scanline.
     void    reset(int width);
 
@@ skipping 38 matching lines @@
             x = 0;
             SkDEBUGCODE(this->validate();)
         }
 
         if (middleCount) {
             SkAlphaRuns::Break(runs, alpha, x, middleCount);
             alpha += x;
             runs += x;
             x = 0;
             do {
-                alpha[0] = SkToU8(alpha[0] + maxValue);
+                alpha[0] = SkToU8(CatchOverflow(alpha[0] + maxValue));
                 int n = runs[0];
                 SkASSERT(n <= middleCount);
                 alpha += n;
                 runs += n;
                 middleCount -= n;
             } while (middleCount > 0);
             SkDEBUGCODE(this->validate();)
             lastAlpha = alpha;
         }
 
@@ skipping 88 matching lines @@
             x -= n;
         }
     }
 
 private:
     SkDEBUGCODE(int fWidth;)
     SkDEBUGCODE(void validate() const;)
 };
 
 #endif