fix cache hint for shrink
fix cache hint for shrink

shrink had UNBUFFERED as a cache hint, but for non-int shrinks it used
affine, so you could get out of order reads

set regular SEQ as the cache hint, and switch to reduce for the final
resize in the non-int case

see https://github.com/jcupitt/libvips/issues/596

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