add runtime option to provide data->imagegenerator factory

gm/mipmap.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "SkCanvas.h"
 #include "SkImage.h"
@@ skipping 11 matching lines @@
 
     paint.setStyle(SkPaint::kStroke_Style);
     for (int i = 0; i < 20; ++i) {
         canvas->drawCircle(-4, 25, 20, paint);
         canvas->translate(25, 0);
     }
     return surface->newImageSnapshot();
 }
 
 static void test_mip(SkCanvas* canvas) {
-    SkAutoTUnref<SkImage> img(make_image());//SkImage::NewFromData(data));
+    SkAutoTUnref<SkImage> img(make_image());//SkImage::NewFromEncoded(data));
 
     SkPaint paint;
     const SkRect dst = SkRect::MakeWH(177, 15);
 
     paint.setTextSize(30);
     SkString str;
     str.printf("scale %g %g", dst.width() / img->width(), dst.height() / img->height());
 //    canvas->drawText(str.c_str(), str.size(), 300, 100, paint);
 
     canvas->translate(20, 20);
@@ skipping 16 matching lines @@
 
     void onDraw(SkCanvas* canvas) override {
         test_mip(canvas);
     }
 
 private:
     typedef skiagm::GM INHERITED;
 };
 DEF_GM( return new MipMapGM; )