add compose-colorfilter

include/core/SkColorFilter.h

Index: include/core/SkColorFilter.h
diff --git a/include/core/SkColorFilter.h b/include/core/SkColorFilter.h
index 929607a398592f8d850590d0f69186b8c25c71f1..bb628053a643527b72d876b8da88cd8302ee6f00 100644
--- a/include/core/SkColorFilter.h
+++ b/include/core/SkColorFilter.h
@@ -123,6 +123,12 @@ public:
     */
     static SkColorFilter* CreateLightingFilter(SkColor mul, SkColor add);
 
+    /** Construct a colorfilter whose effect is to first apply the inner filter and then apply
+     *  the outer filter to the result of the inner's.
+     *  The reference counts for outer and inner are incremented.
+     */
+    static SkColorFilter* CreateComposeFilter(SkColorFilter* outer, SkColorFilter* inner);

[f(malita), 2015/03/02 00:47:07]

I was expecting the arg order to correspond to the order in which the filters
are applied, and found the current order slightly surprising. I presume it's due
to C' = outer(inner(C)), so from that angle it does make sense.

+
     /** A subclass may implement this factory function to work with the GPU backend. If the return
         is non-NULL then the caller owns a ref on the returned object.
      */