add ImageShader, sharing code with its Bitmap cousin

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 47934dd9ad9579b4cdd8b0209039e4e90f3f5035..4104c0e1dc54628d900b4801372cf6b5e872b791 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -413,11 +413,12 @@ public:
     SK_TO_STRING_VIRT()
     SK_DEFINE_FLATTENABLE_TYPE(SkShader)
 
+    // Should be protected, but sometimes a subclass needs to call it on another shader
+    bool computeTotalInverse(const ContextRec&, SkMatrix* totalInverse) const;

[f(malita), 2015/09/15 15:01:41]

Befriend SkBitmapProcShader instead?

+
 protected:
     void flatten(SkWriteBuffer&) const override;
 
-    bool computeTotalInverse(const ContextRec&, SkMatrix* totalInverse) const;
-
     /**
      *  Your subclass must also override contextSize() if it overrides onCreateContext().
      *  Base class impl returns NULL.