vips_resize() uses vips_affine() for upsizing and vips_reduce() for
downsizing. Affine automaticaly does a vips_premultiply() for images
with an alpha channel, but reduce does not. This meant that we could
sometimes premultiply twice.
This patch adds a "premultiplied" flag for affine which turns automatic
premultiuplication off, vips_resize() uses this to block affine's auto
premul feature, and the resize docs are clarified to stress that the
operation does not do premultiplication for you.
See https://github.com/libvips/libvips/issues/1629
ba0dea001d