Make GrGLShaderBuilder check whether GrEffect advertised that it would require the dst color or frag

src/gpu/GrAARectRenderer.cpp

Index: src/gpu/GrAARectRenderer.cpp
diff --git a/src/gpu/GrAARectRenderer.cpp b/src/gpu/GrAARectRenderer.cpp
index 28dae2420fe4281dae24220e17671a9c85e01db5..0b593b9a5f39729f4a5be96d083d16485bf56944 100644
--- a/src/gpu/GrAARectRenderer.cpp
+++ b/src/gpu/GrAARectRenderer.cpp
@@ -220,6 +220,7 @@ private:
     GrRectEffect() : GrEffect() {
         this->addVertexAttrib(kVec4f_GrSLType);
         this->addVertexAttrib(kVec2f_GrSLType);
+        this->setWillReadFragmentPosition();
     }
 
     virtual bool onIsEqual(const GrEffect&) const SK_OVERRIDE { return true; }