move compose-shader into base-class as factory

next steps:
1. this lands
2. update android to call the new factory
3. move SkComposeShader.h into private

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1686803003

Review URL: https://codereview.chromium.org/1686803003

1 files changed, 13 insertions, 1 deletions

diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 297091d542..36256f6f09 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