Add support for reading the dst pixel value in an effect. Use in a new effect for the kDarken xfer …

include/gpu/GrEffect.h

Index: include/gpu/GrEffect.h
===================================================================
--- include/gpu/GrEffect.h	(revision 8448)
+++ include/gpu/GrEffect.h	(working copy)
@@ -138,6 +138,8 @@
     /** Shortcut for textureAccess(index).texture(); */
     GrTexture* texture(int index) const { return this->textureAccess(index).getTexture(); }
 
+    /** Will this effect read the destination pixel value? */
+    bool willReadDst() const { return fWillReadDst; }
 
     int numVertexAttribs() const { return fVertexAttribTypes.count(); }
 
@@ -145,7 +147,6 @@
 
     static const int kMaxVertexAttribs = 2;
 
-
     /** Useful for effects that want to insert a texture matrix that is implied by the texture
         dimensions */
     static inline SkMatrix MakeDivByTextureWHMatrix(const GrTexture* texture) {
@@ -191,7 +192,7 @@
      */
     void addVertexAttrib(GrSLType type);
 
-    GrEffect() : fEffectRef(NULL) {};
+    GrEffect() : fWillReadDst(false), fEffectRef(NULL) {}
 
     /** This should be called by GrEffect subclass factories. See the comment on AutoEffectUnref for
         an example factory function. */
@@ -234,6 +235,13 @@
         return *static_cast<const T*>(&effectRef);
     }
 
+    /**
+     * If the effect subclass will read the destination pixel value then it must call this function
+     * from its constructor. Otherwise, when its generated backend-specific effect class attempts
+     * to generate code that reads the destination pixel it will fail.
+     */
+    void setWillReadDst() { fWillReadDst = true; }
+
 private:
     bool isEqual(const GrEffect& other) const {
         if (&this->getFactory() != &other.getFactory()) {
@@ -265,6 +273,7 @@
 
     SkSTArray<4, const GrTextureAccess*, true>   fTextureAccesses;
     SkSTArray<kMaxVertexAttribs, GrSLType, true> fVertexAttribTypes;
+    bool                                         fWillReadDst;
     GrEffectRef*                                 fEffectRef;
 
     typedef GrRefCnt INHERITED;