Scanline decoding for gifs

include/codec/SkScanlineDecoder.h

Index: include/codec/SkScanlineDecoder.h
diff --git a/include/codec/SkScanlineDecoder.h b/include/codec/SkScanlineDecoder.h
index 61184d644a15d9043e2fef92f3aa1e54a01e21e8..34c66c95f8dd014290ccb6ccd1fbd9f3ab5dcba1 100644
--- a/include/codec/SkScanlineDecoder.h
+++ b/include/codec/SkScanlineDecoder.h
@@ -215,7 +215,9 @@ public:
 
     /**
      *  An enum representing the order in which scanlines will be returned by
-     *  the scanline decoder.
+     *  the scanline decoder.  This should not be called until after we have
+     *  called start().  Specifically, we do not know if gifs are interlaced

[scroggo, 2015/09/02 22:45:48]

From your comments elsewhere, it sounded like it should be fine to move that
check into ReadHeader? I don't like that this sometimes gives the wrong order.

Alternatively, calling this function could read enough of the encoded image to
determine the order.

[msarett, 2015/09/03 17:13:30]

I'm happy add more to ReadHeader().  I agree that this is weird.  I had thought
we had decided in person to punt this issue down the road, but I'll make the
fix.

FYI, this fix will also help with guessing the alpha type.

[scroggo, 2015/09/04 18:42:09]

We might have decided on that, but I think I did not fully understand.
Great!

+     *  until after the call to start().
      */
     SkScanlineOrder getScanlineOrder() const { return this->onGetScanlineOrder(); }