alpha threshold bitmap shader

include/gpu/GrEffect.h

Index: include/gpu/GrEffect.h
diff --git a/include/gpu/GrEffect.h b/include/gpu/GrEffect.h
index 942e580d4c3a8d5d26d2e93d968d52a99065b805..a84810c094101158023a1e947ada5071e274066b 100644
--- a/include/gpu/GrEffect.h
+++ b/include/gpu/GrEffect.h
@@ -204,8 +204,9 @@ public:
 protected:
     /**
      * Subclasses call this from their constructor to register GrTextureAccesses. The effect
-     * subclass manages the lifetime of the accesses (this function only stores a pointer). This
-     * must only be called from the constructor because GrEffects are immutable.
+     * subclass manages the lifetime of the accesses (this function only stores a pointer). The
+     * GrTextureAccess is typically a member field of the GrEffet subclass. This must only be
+     * called from the constructor because GrEffects are immutable.
      */
     void addTextureAccess(const GrTextureAccess* textureAccess);