Scanline decoding for gifs

include/codec/SkScanlineDecoder.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkScanlineDecoder_DEFINED
 #define SkScanlineDecoder_DEFINED
 
@@ skipping 197 matching lines @@
          * determine which scanlines are needed and ask for all of them in
          * a single call to getScanlines().
          *
          * Interlaced pngs are an example.
          */
         kNone_SkScanlineOrder,
     };
 
     /**
      *  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(); }
 
     /**
      *  Returns the y-coordinate of the next row to be returned by the scanline
      *  decoder.  This will be overridden in the case of
      *  kOutOfOrder_SkScanlineOrder and should be unnecessary in the case of
      *  kNone_SkScanlineOrder.
      */
     int getY() const {
@@ skipping 50 matching lines @@
         // Calling the virtual method also means we do not double count
         // countLines.
         return this->onGetScanlines(storage.get(), countLines, 0);
     }
 
     virtual SkCodec::Result onGetScanlines(void* dst, int countLines,
                                                     size_t rowBytes) = 0;
 
 };
 #endif // SkScanlineDecoder_DEFINED