See Section 23. Graphic Control Extension of the spec:
https://www.w3.org/Graphics/GIF/spec-gif89a.txt

Change-Id: Ie78b4ff4aa97e1b332ade67ae4fa25f7c0770610
Reviewed-on: https://go-review.googlesource.com/22547Reviewed-by: Rob Pike <r@golang.org>