Add the ability to decode a subset to SkCodec.

dm/DMSrcSink.h

Index: dm/DMSrcSink.h
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index b7c28edf006f17e1c32504364c7d2456fff8d97a..b80c7c9a4bd5b837ebd88092abb267f83cd0ef94 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -99,6 +99,7 @@ public:
         kScanline_Mode,
         kScanline_Subset_Mode,
         kStripe_Mode, // Tests the skipping of scanlines
+        kSubset_Mode, // For codecs that support subsets directly.

[djsollen, 2015/07/20 14:29:24]

Is there any method for the caller to negotiate with the API?  For example, can
I give it a requested subset and it return a closest match?

[scroggo, 2015/07/20 14:47:01]

That's a good idea - much like getScaledDimensions. That said, I'm not sure what
the closest match would be. libwebp decided to snap odd numbers to the left, but
we could just as easily snap odd numbers right. And what do we do about the
width and height? (i.e. if we changed top or left, do we change width or height
so the right/bottom is unchanged? or do we leave them unchanged so that
right/bottom has changed?)

[djsollen, 2015/07/20 16:54:34]

I would err on the side of giving them too many pixels and not too few as the
best match.

[scroggo, 2015/07/20 18:55:54]

Done.

     };
     enum DstColorType {
         kGetFromCanvas_DstColorType,