Use GrVertexWriter in AA convex path renderer

Bug: skia:
Change-Id: I7f4107ee64079a00302fc3fa9b81fba73b5f3219
Reviewed-on: https://skia-review.googlesource.com/c/179989
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
diff --git a/src/gpu/GrVertexWriter.h b/src/gpu/GrVertexWriter.h
index e215793..de4c78f 100644
--- a/src/gpu/GrVertexWriter.h
+++ b/src/gpu/GrVertexWriter.h
@@ -41,6 +41,9 @@
         return Conditional<T>(condition, value);
     }
 
+    template <typename T>
+    struct Skip {};
+
     template <typename T, typename... Args>
     void write(const T& val, const Args&... remainder) {
         static_assert(std::is_pod<T>::value, "");
@@ -79,6 +82,12 @@
         this->write(remainder...);
     }
 
+    template <typename T, typename... Args>
+    void write(const Skip<T>& val, const Args&... remainder) {
+        fPtr = SkTAddOffset<void>(fPtr, sizeof(T));
+        this->write(remainder...);
+    }
+
     template <typename... Args>
     void write(const Sk4f& vector, const Args&... remainder) {
         float buffer[4];