Fix image size off by one
Fix image size off by one

Note this also saves loading the data into an `image.Image` to
calculate resultant image height, so it’s a slight performance bump.

See discussion here:
https://github.com/DAddYE/vips/issues/16#issuecomment-86195169

foresmac/vip,vokal/vip,WorkHorseIndustries/vip