fix TriColorShader to respect the paint's alpha

src/core/SkDraw.cpp

Index: src/core/SkDraw.cpp
diff --git a/src/core/SkDraw.cpp b/src/core/SkDraw.cpp
index a74e3c0f798123845c718fb717a30bba6ca9e4f9..305cdd389f9a5ec0ed95f0c0efb0b766e5d07f25 100644
--- a/src/core/SkDraw.cpp
+++ b/src/core/SkDraw.cpp
@@ -2432,6 +2432,9 @@ size_t SkTriColorShader::contextSize() const {
     return sizeof(TriColorShaderContext);
 }
 void SkTriColorShader::TriColorShaderContext::shadeSpan(int x, int y, SkPMColor dstC[], int count) {
+    const unsigned alpha = this->getPaintAlpha();
+    const int alphaScale = alpha + (alpha >> 7);

[scroggo, 2014/05/05 21:15:02]

From the comments in SkColorPriv, it looks like this is the more accurate
version of SkAlpha255To256; would it make sense to provide both as helpers?

[reed1, 2014/05/05 21:25:15]

Done.

+
     SkPoint src;
 
     for (int i = 0; i < count; i++) {
@@ -2450,9 +2453,15 @@ void SkTriColorShader::TriColorShaderContext::shadeSpan(int x, int y, SkPMColor
             scale0 = 0;
         }
 
+        if (256 != alphaScale) {
+            scale0 = (scale0 * alphaScale) >> 8;

[scroggo, 2014/05/05 21:15:02]

Why not use SkAlphaMul?

[reed1, 2014/05/05 21:25:15]

Done.

+            scale1 = (scale1 * alphaScale) >> 8;
+            scale2 = (scale2 * alphaScale) >> 8;
+        }
+
         dstC[i] = SkAlphaMulQ(fColors[0], scale0) +
-        SkAlphaMulQ(fColors[1], scale1) +
-        SkAlphaMulQ(fColors[2], scale2);
+                  SkAlphaMulQ(fColors[1], scale1) +
+                  SkAlphaMulQ(fColors[2], scale2);
     }
 }