more tuning of vipsthumbnail
more tuning of vipsthumbnail

leave at least a factor of two after shrink-on-load

helps hide aliasing

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