image/gif: reject a GIF image if frame bounds larger than image bounds

The GIF89a spec says: "Each image must fit within the
boundaries of the Logical Screen, as defined in the
Logical Screen Descriptor." Also, do not accept
GIFs which have too much data for the image size.

R=nigeltao, jra, r
CC=bradfitz, golang-dev
https://codereview.appspot.com/7602045

Committer: Rob Pike <r@golang.org>