Use new jpeg_crop_scanlines() API to optimize jpeg subset decodes

src/codec/SkJpegCodec.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkJpegCodec_DEFINED
 #define SkJpegCodec_DEFINED
 
@@ skipping 97 matching lines @@
     bool onSkipScanlines(int count) override;
 
     SkAutoTDelete<JpegDecoderMgr> fDecoderMgr;
     // We will save the state of the decompress struct after reading the header.
     // This allows us to safely call onGetScaledDimensions() at any time.
     const int                     fReadyState;
 
     // scanline decoding
     SkAutoTMalloc<uint8_t>     fStorage;    // Only used if sampling is needed
     uint8_t*                   fSrcRow;     // Only used if sampling is needed
+    // libjpeg-turbo provides some subsetting.  In the case that libjpeg-turbo
+    // cannot take the exact the subset that we need, we will use the swizzler
+    // to further subset the output from libjpeg-turbo.
+    SkIRect                    fSwizzlerSubset;
     SkAutoTDelete<SkSwizzler>  fSwizzler;
     
     typedef SkCodec INHERITED;
 };
 
 #endif