image/gif: expose disposal, bg index and Config

image/gif: expose disposal, bg index and Config

The background index in the global palette (located in the image.Config)
is necessary for interpreting GIF frames properly

Frame disposal information is necessary for interpreting GIF frames in
the context of a sequence (or animation)

Removes decoder.flags as it can be a local variable

Change-Id: I6790a7febf6ba0859175c834c807bc6413e6b194
Reviewed-on: https://go-review.googlesource.com/4620
Reviewed-by: Nigel Tao <nigeltao@golang.org>