Fix alpha computation in SkArithmeticMode.

gm/arithmode.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "SkCanvas.h"
 #include "SkColorPriv.h"
@@ skipping 11 matching lines @@
     bm.eraseColor(SK_ColorTRANSPARENT);
     return bm;
 }
 
 static SkBitmap make_src() {
     SkBitmap bm = make_bm();
     SkCanvas canvas(bm);
     SkPaint paint;
     SkPoint pts[] = { {0, 0}, {SkIntToScalar(WW), SkIntToScalar(HH)} };
     SkColor colors[] = {
-        SK_ColorBLACK, SK_ColorGREEN, SK_ColorCYAN,
-        SK_ColorRED, SK_ColorMAGENTA, SK_ColorWHITE
+        SK_ColorTRANSPARENT, SK_ColorGREEN, SK_ColorCYAN,
+        SK_ColorRED, SK_ColorMAGENTA, SK_ColorWHITE,
     };
     SkShader* s = SkGradientShader::CreateLinear(pts, colors, NULL, SK_ARRAY_COUNT(colors),
                                                  SkShader::kClamp_TileMode);
     paint.setShader(s)->unref();
     canvas.drawPaint(paint);
     return bm;
 }
 
 static SkBitmap make_dst() {
     SkBitmap bm = make_bm();
@@ skipping 86 matching lines @@
     }
 
 private:
     typedef GM INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 static skiagm::GM* MyFactory(void*) { return new ArithmodeGM; }
 static skiagm::GMRegistry reg(MyFactory);