fix TriColorShader to respect the paint's alpha

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 163 matching lines @@
 
     In debugging, asserts that alpha is 0..255
 */
 static inline unsigned SkAlpha255To256(U8CPU alpha) {
     SkASSERT(SkToU8(alpha) == alpha);
     // this one assues that blending on top of an opaque dst keeps it that way
     // even though it is less accurate than a+(a>>7) for non-opaque dsts
     return alpha + 1;
 }
 
+/**
+ *  Turn a 0..255 value into a 0..256 value, rounding up if the value is >= 0x80.
+ *  This is slightly more accurate than SkAlpha255To256.
+ */
+static inline unsigned Sk255To256(U8CPU value) {
+    SkASSERT(SkToU8(value) == value);
+    return value + (value >> 7);
+}
+
 /** Multiplify value by 0..256, and shift the result down 8
     (i.e. return (value * alpha256) >> 8)
  */
 #define SkAlphaMul(value, alpha256)     (SkMulS16(value, alpha256) >> 8)
 
 //  The caller may want negative values, so keep all params signed (int)
 //  so we don't accidentally slip into unsigned math and lose the sign
 //  extension when we shift (in SkAlphaMul)
 static inline int SkAlphaBlend(int src, int dst, int scale256) {
     SkASSERT((unsigned)scale256 <= 256);
@@ skipping 849 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