Use new jpeg_crop_scanlines() API to optimize jpeg subset decodes

src/codec/SkJpegCodec.h

Index: src/codec/SkJpegCodec.h
diff --git a/src/codec/SkJpegCodec.h b/src/codec/SkJpegCodec.h
index 06685cfd4c71a5bf9ab6ea523060be4b65f43558..bb5ce75375fa899c238dfd0dbeeaafcb31814242 100644
--- a/src/codec/SkJpegCodec.h
+++ b/src/codec/SkJpegCodec.h
@@ -115,6 +115,10 @@ private:
     // 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;

[scroggo, 2016/02/22 15:55:03]

Should we initialize this at construction/when we're not using it (or comment
that we deliberately didn't do so for performance; though it seems like
initializing four ints at construction/when unused is not so bad?)?

[msarett, 2016/02/22 17:38:16]

Sure I'm fine with initializing at construction.

     SkAutoTDelete<SkSwizzler>  fSwizzler;
     
     typedef SkCodec INHERITED;