Change SkUnitScalarClampToByte to more accurate implementation.

include/core/SkColorPriv.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 SkColorPriv_DEFINED
 #define SkColorPriv_DEFINED
 
@@ skipping 206 matching lines @@
     SkASSERT((int16_t)src == src);
     SkASSERT((int16_t)dst == dst);
     SkASSERT((uint8_t)alpha == alpha);
 
     int prod = (src - dst) * alpha + 128;
     prod = (prod + (prod >> 8)) >> 8;
     return dst + prod;
 }
 
 static inline U8CPU SkUnitScalarClampToByte(SkScalar x) {
-    if (x < 0) {
-        return 0;
-    }
-    if (x >= SK_Scalar1) {
-        return 255;
-    }
-    return SkScalarToFixed(x) >> 8;
+    return static_cast<U8CPU>(SkScalarPin(x, 0, 1) * 255 + 0.5);
 }
 
 #define SK_R16_BITS     5
 #define SK_G16_BITS     6
 #define SK_B16_BITS     5
 
 #define SK_R16_SHIFT    (SK_B16_BITS + SK_G16_BITS)
 #define SK_G16_SHIFT    (SK_B16_BITS)
 #define SK_B16_SHIFT    0
 
@@ skipping 814 matching lines @@
     int srcG = SkColorGetG(src);
     int srcB = SkColorGetB(src);
 
     for (int i = 0; i < width; i++) {
         dst[i] = SkBlendLCD16Opaque(srcR, srcG, srcB, dst[i], mask[i],
                                     opaqueDst);
     }
 }
 
 #endif