Add luminance mask transfer modes.

include/core/SkXfermode.h

Index: include/core/SkXfermode.h
diff --git a/include/core/SkXfermode.h b/include/core/SkXfermode.h
index ef00f71b75ecb95bf4a900718efb79356e0a1b35..991fee500dc479e571b680c17cacad9a4d43f35a 100644
--- a/include/core/SkXfermode.h
+++ b/include/core/SkXfermode.h
@@ -127,7 +127,14 @@ public:
         kSaturation_Mode,
         kColor_Mode,
         kLuminosity_Mode,
-        kLastMode = kLuminosity_Mode
+
+        // Luminance mask modes, to support SVG & CSS luminance masks.
+        // http://www.w3.org/TR/SVG/masking.html#Masking
+        // http://www.w3.org/TR/css-masking/
+        kSrcInLum_Mode,

[reed1, 2013/08/20 17:21:35]

Can we initially introduce this xfermode w/o adding it to public SkXfermode.h's
enum? See SkArithmeticMode.h as an example.

+        kDstInLum_Mode,
+
+        kLastMode = kDstInLum_Mode
     };
 
     /**