Add dual source blending support for proper blending with coverage.

Review URL: http://codereview.appspot.com/4535088/



git-svn-id: http://skia.googlecode.com/svn/trunk@1390 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/src/GrGLInterface.cpp b/gpu/src/GrGLInterface.cpp
index 0825a3f..5ecf8eb 100644
--- a/gpu/src/GrGLInterface.cpp
+++ b/gpu/src/GrGLInterface.cpp
@@ -333,6 +333,15 @@
         }
     }
 
+    // Dual source blending
+    if (kDesktop_GrGLBinding == fBindingsExported  &&
+        (has_gl_extension_from_string("GL_ARB_blend_func_extended", ext) ||
+         (3 < major) || (3 == major && 3 <= minor))) {
+        if (NULL == fBindFragDataLocationIndexed) {
+            return false;
+        }
+    }
+
     return true;
 }