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

src/gpu/GrAARectRenderer.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrAARectRenderer.h"
 #include "GrRefCnt.h"
 #include "GrGpu.h"
@@ skipping 202 matching lines @@
 
     private:
         typedef GrGLEffect INHERITED;
     };
 
 
 private:
     GrRectEffect() : GrEffect() {
         this->addVertexAttrib(kVec4f_GrSLType);
         this->addVertexAttrib(kVec2f_GrSLType);
+        this->setWillReadFragmentPosition();
     }
 
     virtual bool onIsEqual(const GrEffect&) const SK_OVERRIDE { return true; }
 
     GR_DECLARE_EFFECT_TEST;
 
     typedef GrEffect INHERITED;
 };
 
 
@@ skipping 461 matching lines @@
     // The innermost rect has full coverage
     verts += 8 * vsize;
     for (int i = 0; i < 4; ++i) {
         *reinterpret_cast<GrColor*>(verts + i * vsize) = 0;
     }
 
     target->setIndexSourceToBuffer(indexBuffer);
     target->drawIndexed(kTriangles_GrPrimitiveType,
                         0, 0, 16, aaStrokeRectIndexCount());
 }