[cc] Add SkBlendMode::kDstIn support to cc::Layer

cc/output/shader.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/shader.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <vector>
@@ skipping 706 matching lines @@
     case BLEND_MODE_COLOR:
     case BLEND_MODE_LUMINOSITY:
       return kFunctionLum;
     default:
       return std::string();
   }
 }
 
 std::string FragmentShader::GetBlendFunction() const {
   return "vec4 Blend(vec4 src, vec4 dst) {"
-         "    vec4 result;"
-         "    result.a = src.a + (1.0 - src.a) * dst.a;" +
-         GetBlendFunctionBodyForRGB() +
+         "    vec4 result;" +
+         GetBlendFunctionBodyForAlpha() + GetBlendFunctionBodyForRGB() +
          "    return result;"
          "}";
 }
 
+std::string FragmentShader::GetBlendFunctionBodyForAlpha() const {
+  if (blend_mode_ == BLEND_MODE_DESTINATION_IN)
+    return "result.a = src.a * dst.a;";
+  else
+    return "result.a = src.a + (1.0 - src.a) * dst.a;";
+}
+
 std::string FragmentShader::GetBlendFunctionBodyForRGB() const {
   switch (blend_mode_) {
     case BLEND_MODE_NORMAL:
       return "result.rgb = src.rgb + dst.rgb * (1.0 - src.a);";
+    case BLEND_MODE_DESTINATION_IN:
+      return "result.rgb = dst.rgb * src.a;";
     case BLEND_MODE_SCREEN:
       return "result.rgb = src.rgb + (1.0 - src.rgb) * dst.rgb;";
     case BLEND_MODE_LIGHTEN:
       return "result.rgb = max((1.0 - src.a) * dst.rgb + src.rgb,"
              "                 (1.0 - dst.a) * src.rgb + dst.rgb);";
     case BLEND_MODE_OVERLAY:
       return "result.rgb = hardLight(dst, src);";
     case BLEND_MODE_DARKEN:
       return "result.rgb = min((1.0 - src.a) * dst.rgb + src.rgb,"
              "                 (1.0 - dst.a) * src.rgb + dst.rgb);";
@@ skipping 246 matching lines @@
       else
         SRC("gl_FragColor = ApplyBlendMode(texColor, 0.0);");
       break;
   }
   source += "}\n";
 
   return header + source;
 }
 
 }  // namespace cc