Fix EffectKey for XferEffect

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 674078ef3a1edebb3d63f69cef1beb4166a27bf4..78b85daa3e4e1ab546b87e6d33b9b89b8269692e 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -1079,7 +1079,10 @@ public:
         }
 
         static inline EffectKey GenKey(const GrDrawEffect& drawEffect, const GrGLCaps&) {
-            return drawEffect.castEffect<XferEffect>().mode();
+            // The background may come from the dst or from a texture.
+            int numTextures = (*drawEffect.effect())->numTextures();
+            SkASSERT(numTextures <= 1);
+            return (drawEffect.castEffect<XferEffect>().mode() << 1) | numTextures;
         }
 
     private: