Fix the rendering error of SkDraw::drawVertices in gpu path for solid color

include/core/SkCanvas.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCanvas_DEFINED
 #define SkCanvas_DEFINED
 
@@ skipping 984 matching lines @@
     */
     virtual void drawPicture(SkPicture& picture);
 
     enum VertexMode {
         kTriangles_VertexMode,
         kTriangleStrip_VertexMode,
         kTriangleFan_VertexMode
     };
 
     /** Draw the array of vertices, interpreted as triangles (based on mode).
+
+        If both textures and vertex-colors are NULL, it strokes hairlines with
+        the paint's color. This behavior is a useful debugging mode to visualize
+        the mesh.
+
         @param vmode How to interpret the array of vertices
         @param vertexCount The number of points in the vertices array (and
                     corresponding texs and colors arrays if non-null)
         @param vertices Array of vertices for the mesh
         @param texs May be null. If not null, specifies the coordinate
                     in _texture_ space (not uv space) for each vertex.
         @param colors May be null. If not null, specifies a color for each
                       vertex, to be interpolated across the triangle.
         @param xmode Used if both texs and colors are present. In this
                     case the colors are combined with the texture using mode,
@@ skipping 486 matching lines @@
     bool asROBitmap(SkBitmap*) const;
 
 private:
     SkBitmap    fBitmap;    // used if peekPixels() fails
     const void* fAddr;      // NULL on failure
     SkImageInfo fInfo;
     size_t      fRowBytes;
 };
 
 #endif