Replace fWillReadFragmentPosition with a bitfield

src/gpu/GrFragmentProcessor.cpp

Index: src/gpu/GrFragmentProcessor.cpp
diff --git a/src/gpu/GrFragmentProcessor.cpp b/src/gpu/GrFragmentProcessor.cpp
index 4c3d0eee47816f42eed8470114e51e775b74365b..495a7b3a33454f759304c017ecdb325e9722a6f3 100644
--- a/src/gpu/GrFragmentProcessor.cpp
+++ b/src/gpu/GrFragmentProcessor.cpp
@@ -96,9 +96,7 @@ int GrFragmentProcessor::registerChildProcessor(const GrFragmentProcessor* child
     int index = fChildProcessors.count();
     fChildProcessors.push_back(SkRef(child));
 
-    if (child->willReadFragmentPosition()) {
-        this->setWillReadFragmentPosition();
-    }
+    this->combineRequiredFeatures(*child);
 
     if (child->usesLocalCoords()) {
         fUsesLocalCoords = true;