Refactor resize filter to go faster

src/core/SkBitmapFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmapFilter.h"
 #include "SkRTConf.h"
 #include "SkTypes.h"
 
 #include <string.h>
 
 // These are the per-scanline callbacks that are used when we must resort to
 // resampling an image as it is blitted.  Typically these are used only when
 // the image is rotated or has some other complex transformation applied.
 // Scaled images will usually be rescaled directly before rasterization.
 
 SK_CONF_DECLARE(const char *, c_bitmapFilter, "bitmap.filter", "mitchell", "Which scanline bitmap filter to use [mitchell, lanczos, hamming, gaussian, triangle, box]");
 
 SkBitmapFilter *SkBitmapFilter::Allocate() {
     if (!strcmp(c_bitmapFilter, "mitchell")) {
-        return new SkMitchellFilter(1.f / 3.f, 1.f / 3.f);
+        return new SkMitchellFilter;
     } else if (!strcmp(c_bitmapFilter, "lanczos")) {
         return new SkLanczosFilter;
     } else if (!strcmp(c_bitmapFilter, "hamming")) {
         return new SkHammingFilter;
     } else if (!strcmp(c_bitmapFilter, "gaussian")) {
         return new SkGaussianFilter(2);
     } else if (!strcmp(c_bitmapFilter, "triangle")) {
         return new SkTriangleFilter;
     } else if (!strcmp(c_bitmapFilter, "box")) {
         return new SkBoxFilter;
     } else {
         SkDEBUGFAIL("Unknown filter type");
     }
 
     return nullptr;
 }