handle deferred clear codes

These show up fairly frequently in the wild; ImageMagick handles them
correctly while ImageIO throws an error and JAI and GifDecoder only load
the top portion of the frames. Ex:
http://24.media.tumblr.com/tumblr_mceuhx0qgj1r5hubco1_500.gif
http://uberhumor.com/wp-content/uploads/2012/09/Gyvdz2.gif
https://i.chzbgr.com/completestore/12/10/23/H0I7k2e8M069BDdrEDb5ww2.gif