fix TriColorShader to respect the paint's alpha

results can be seen in new gm: vertices_80

BUG=skia:
R=scroggo@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/270023002

git-svn-id: http://skia.googlecode.com/svn/trunk@14581 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkColorPriv.h

@@ -181,6 +181,15 @@ static inline unsigned SkAlpha255To256(U8CPU alpha) {
     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)
  */

src/core/SkDraw.cpp

@@ -2432,6 +2432,8 @@ size_t SkTriColorShader::contextSize() const {
     return sizeof(TriColorShaderContext);
 }
 void SkTriColorShader::TriColorShaderContext::shadeSpan(int x, int y, SkPMColor dstC[], int count) {
+    const int alphaScale = Sk255To256(this->getPaintAlpha());
+
     SkPoint src;
 
     for (int i = 0; i < count; i++) {
@@ -2450,6 +2452,12 @@ void SkTriColorShader::TriColorShaderContext::shadeSpan(int x, int y, SkPMColor
             scale0 = 0;
         }
 
+        if (256 != alphaScale) {
+            scale0 = SkAlphaMul(scale0, alphaScale);
+            scale1 = SkAlphaMul(scale1, alphaScale);
+            scale2 = SkAlphaMul(scale2, alphaScale);
+        }
+
         dstC[i] = SkAlphaMulQ(fColors[0], scale0) +
                   SkAlphaMulQ(fColors[1], scale1) +
                   SkAlphaMulQ(fColors[2], scale2);