Source: http://java2s.com/Code/Java/2D-Graphics-GUI/AnimatedGifEncoder.htm
This only affects the default decode path where either a GIF or a Bitmap is an acceptable result. The GIF only path will still return GifDrawables for single frame GIFs. Fixes #160.
The GCE is an optional extension that may or may not be included per frame (it's slightly more complicated than this), we shouldn't throw if it isn't present. Fixes #134.