Use GLib's i18n support instead of copying and pasting that
logic into its own header. This deprecates the vips/intl.h
header in favour of glib/gi18n.h.
We had two API calls, vips_image_copy_fields() and vips_demand_hint().
They are now combined in the single vips_image_pipeline() call.
All operations are now slightly smaller and simpler.