GrFP can express distance vector field req., program builder declares variable for it

include/gpu/GrFragmentProcessor.h

Index: include/gpu/GrFragmentProcessor.h
diff --git a/include/gpu/GrFragmentProcessor.h b/include/gpu/GrFragmentProcessor.h
index b8ebeca30320b3f8a8e21e33e4a16395ef8d5bf2..4997885eebda8528666146af71adc4b69a7181cf 100644
--- a/include/gpu/GrFragmentProcessor.h
+++ b/include/gpu/GrFragmentProcessor.h
@@ -59,6 +59,7 @@ public:
 
     GrFragmentProcessor()
         : INHERITED()
+        , fUsesDistanceVectorField(false)
         , fUsesLocalCoords(false)
         , fNumTexturesExclChildren(0)
         , fNumBuffersExclChildren(0)
@@ -104,6 +105,9 @@ public:
     /** Do any of the coordtransforms for this processor require local coords? */
     bool usesLocalCoords() const { return fUsesLocalCoords; }
 
+    /** Does this FP need a vector to the nearest edge? */
+    bool usesDistanceVectorField() const { return fUsesDistanceVectorField; }
+
     /** Returns true if this and other processor conservatively draw identically. It can only return
         true when the two processor are of the same subclass (i.e. they return the same object from
         from getFactory()).
@@ -167,6 +171,11 @@ protected:
      */
     virtual void onComputeInvariantOutput(GrInvariantOutput* inout) const = 0;
 
+    /* Sub-classes should set this to true in their constructors if they need access to a distance
+     * vector field to the nearest edge
+     */
+    bool fUsesDistanceVectorField;
+
 private:
     void notifyRefCntIsZero() const final;