Fix the issue of alpha = 254 instead of 255

src/core/SkAntiRun.h

Index: src/core/SkAntiRun.h
diff --git a/src/core/SkAntiRun.h b/src/core/SkAntiRun.h
index 8214e28d5b7ee8d8575c082d31b77e9f49388d5a..5ac0e62a6ce99129f356c43709b14c45c5fb2712 100644
--- a/src/core/SkAntiRun.h
+++ b/src/core/SkAntiRun.h
@@ -21,6 +21,11 @@ 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 {
@@ -79,7 +84,7 @@ public:
             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;