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.
You must now include vips7 support explicitly with
#include <vips/vips7compat.h>
in your code just after including `vips.h`.
The old vips7 names, such as `Rect`, were starting to cause problems
with other packages like opencv.