The LZW decode step will now copy and pad LZW data out to 4 bytes if there are
less than 4 bytes remaining in the buffer. This means it will now also work when
the total size of the LZW image data is less than 4 bytes.
The sniff method is intended to be used for content sniffing. It should be a
cheap test to rapidly rule out whether a candidate image can be successfully
decoded. For the GIF and PNG implementations it simply attempts to decode the
image header, returning true if successful and false if not.
Also:
- Define the GIFLoadingContext structure.
- The load_gif_impl function now returns load operation success, and takes a
reference to a GIFLoadingContext as input.
- Implement GIFImageDecoderPlugin::bitmap which calls onto load_gif_impl.
