Use new jpeg_crop_scanlines() API to optimize jpeg subset decodes

dm/DMSrcSink.cpp

Index: dm/DMSrcSink.cpp
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index 7ccaf223fc3a32b8e2baa867dcd575454332b6ec..bd881e8381739a1c723f8014ed0859fa0a747473 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -522,6 +522,31 @@ Error CodecSrc::draw(SkCanvas* canvas) const {
             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;