just one sink_disc() for magicksave
just one sink_disc() for magicksave

we used to cut the input image into tiles, then sink_disc() for each one
... instead, make a single sink_disc() and chop in _generate

simpler and faster

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