Use new jpeg_crop_scanlines() API to optimize jpeg subset decodes

dm/DMSrcSink.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 "DMSrcSink.h"
 #include "SkAndroidCodec.h"
 #include "SkCodec.h"
@@ skipping 504 matching lines @@
                 // Skip a stripe
                 const int linesToSkip = SkTMin(stripeHeight, height - (i + 1) * stripeHeight);
                 if (linesToSkip > 0) {
                     codec->skipScanlines(linesToSkip);
                 }
             }
             premultiply_if_necessary(bitmap);
             canvas->drawBitmap(bitmap, 0, 0);
             break;
         }
+        case kCroppedScanline_Mode: {
+            const int width = decodeInfo.width();
+            const int height = decodeInfo.height();
+            // This value is chosen because, as we move across the image, it will sometimes
+            // align with the jpeg block sizes and it will sometimes not.  This allows us
+            // to test interestingly different code paths in the implementation.
+            const int tileSize = 36;
+
+            SkCodec::Options opts;
+            SkIRect subset;
+            for (int x = 0; x < width; x += tileSize) {
+                subset = SkIRect::MakeXYWH(x, 0, SkTMin(tileSize, width - x), height);
+                opts.fSubset = &subset;
+                if (SkCodec::kSuccess != codec->startScanlineDecode(decodeInfo, &opts,
+                        colorPtr, colorCountPtr)) {
+                    return "Could not start scanline decoder.";
+                }
+
+                codec->getScanlines(bitmap.getAddr(x, 0), height, bitmap.rowBytes());
+            }
+
+            premultiply_if_necessary(bitmap);
+            canvas->drawBitmap(bitmap, 0, 0);
+            break;
+        }
         case kSubset_Mode: {
             // Arbitrarily choose a divisor.
             int divisor = 2;
             // Total width/height of the image.
             const int W = codec->getInfo().width();
             const int H = codec->getInfo().height();
             if (divisor > W || divisor > H) {
                 return Error::Nonfatal(SkStringPrintf("Cannot codec subset: divisor %d is too big "
                                                       "for %s with dimensions (%d x %d)", divisor,
                                                       fPath.c_str(), W, H));
@@ skipping 872 matching lines @@
             skr.visit<void>(i, drawsAsSingletonPictures);
         }
         SkAutoTUnref<SkPicture> macroPic(macroRec.endRecordingAsPicture());
 
         canvas->drawPicture(macroPic);
         return check_against_reference(bitmap, src, fSink);
     });
 }
 
 }  // namespace DM