Adds methods to determine whether an image is animated, how many times
the animation loops, the number of frames, and to get individual frames.
Implements stubs of these methods for PNGImageDecoderPlugin and
GIFImageDecoderPlugin.
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.
