image/gif: don't assume Encode src's origin is (0, 0)
When gif.Encode is given an "m image.Image" argument that isn't an
*image.Paletted, it creates a temporary *image.Paletted (called pm) that
is intended to be a copy of this image, only with fewer colors.
That creation process, and specifically the opts.Drawer.Draw call that
does the copy, incorrectly assumed that m.Bounds().Min is the zero point
(0, 0). This commit fixes that.