Use new jpeg_crop_scanlines() API to optimize jpeg subset decodes

dm/DMSrcSink.h

Index: dm/DMSrcSink.h
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index a987c7d45d544a782c605c5bd72e28e56bd2b157..92c629ffbcc3c74ffe92a6ac6d76f39ddbca7b6d 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -109,7 +109,7 @@ public:
         // without doubling the size of our test suite.
         kCodecZeroInit_Mode,
         kScanline_Mode,
-        kStripe_Mode, // Tests the skipping of scanlines
+        kTile_Mode, // Tests jpeg subset decoding optimizations

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

"Tile" doesn't seem like it describes this. kPartialScanline? kCroppedScanline?

Also, have we potentially lost some useful coverage by not testing other decodes
in stripe mode?

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

You're right, I think I've gone too far.

My concern is that the kCroppedScanline test is quite slow, and there are
sporadic complaints about the codec tests running for too long.  But I think
it's ok to keep the stripe test and only run the kCroppedScanline test for
jpegs.

         kSubset_Mode, // For codecs that support subsets directly.
         kGen_Mode,    // Test SkCodecImageGenerator (includes YUV)
     };