Use vertex attribute for text color in GrBitmapTextContext to make batching more likely

src/gpu/GrBitmapTextContext.h

Index: src/gpu/GrBitmapTextContext.h
diff --git a/src/gpu/GrBitmapTextContext.h b/src/gpu/GrBitmapTextContext.h
index 23248ebf65eee7c15d99a605951a2c5c9f37f5c8..51193c82f99d507d0261cf275d6af32db03a7bb0 100644
--- a/src/gpu/GrBitmapTextContext.h
+++ b/src/gpu/GrBitmapTextContext.h
@@ -45,7 +45,7 @@ private:
         kDefaultRequestedVerts   = kDefaultRequestedGlyphs * 4,
     };
 
-    SkPoint*                fVertices;
+    void*                   fVertices;
     int32_t                 fMaxVertices;
     GrTexture*              fCurrTexture;
     int                     fCurrVertex;