vips_resize() used to do most of a downsize with vips_shrink() and the
final 200 - 300% with vips_reduce(). This was correct for lanczos2/3,
but not right for linear/cubic, which need more shrink and less
reduce to avoid aliasing.
This patch makes vips_resize() leave the final 100 - 200% to
vips_reduce() for linear/cubic, and leave everything to reduce for
nearest.