move compose-shader into base-class as factory

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 297091d54206fe64b427f5760c70bb8a07086d81..36256f6f09e4e73cc9d7e22d31982e6b1aecfc07 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -323,7 +323,7 @@ public:
      *  the colorfilter.
      */
     SkShader* newWithColorFilter(SkColorFilter*) const;
-    
+
     //////////////////////////////////////////////////////////////////////////
     //  Factory methods for stock shaders
     
@@ -338,6 +338,18 @@ public:
      */
     static SkShader* CreateColorShader(SkColor);
 
+    static SkShader* CreateComposeShader(SkShader* dst, SkShader* src, SkXfermode::Mode);
+
+    /**
+     *  Create a new compose shader, given shaders dst, src, and a combining xfermode mode.
+     *  The xfermode is called with the output of the two shaders, and its output is returned.
+     *  If xfer is null, SkXfermode::kSrcOver_Mode is assumed.
+     *
+     *  Ownership of the shaders, and the xfermode if not null, is not transfered, so the caller
+     *  is still responsible for managing its reference-count for those objects.
+     */
+    static SkShader* CreateComposeShader(SkShader* dst, SkShader* src, SkXfermode* xfer);
+
     /** Call this to create a new shader that will draw with the specified bitmap.
      *
      *  If the bitmap cannot be used (e.g. has no pixels, or its dimensions