GIF's bounds.

Also change the implicit Config Width and Height to be the
Rectangle.Max, not the Dx and Dy, of the first frame's bounds. For the
case where the first frame's bounds is something like (5,5)-(8,8), the
overall width should be 8, not 3.

Change-Id: I3affc484f5e32941a36f15517a92ca8d189d9c22
Reviewed-on: https://go-review.googlesource.com/9465Reviewed-by: Rob Pike <r@golang.org>