fix webp anim load
fix webp anim load

we were dropping the first frame and duplicating the last frame, thanks
to a < / <= mixup

see https://github.com/libvips/libvips/issues/1261

lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips