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

src/gpu/GrPrimitiveProcessor.h

Index: src/gpu/GrPrimitiveProcessor.h
diff --git a/src/gpu/GrPrimitiveProcessor.h b/src/gpu/GrPrimitiveProcessor.h
index 0d5280fbd3ed44703d6a37068e5976f09cd4e547..d27a6f31c78d67390f42c06fc4783175b79da6b1 100644
--- a/src/gpu/GrPrimitiveProcessor.h
+++ b/src/gpu/GrPrimitiveProcessor.h
@@ -221,6 +221,10 @@ public:
         return 0.0;
     }
 
+    /* Sub-class should override and return true if this primitive processor implements the distance
+     * vector field, a field of vectors to the nearest point in the edge of the shape.  */
+    virtual bool implementsDistanceVector() const { return false; }
+
 protected:
     GrPrimitiveProcessor() : fVertexStride(0) {}