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.
tell buffer and target savers the file format
Currently, buffer and target savers are not told the format they should
write.
This is usually OK (the JPEG saver already knows it should write JPEG),
but some savers can write several formats, and these currently need an
extra parameter.
For example:
```ruby
buf = x.write_to_buffer ".bmp", format: "bmp"
```
The first ".bmp" gets libvips to pick magicksave, the second
`format:` param is necessary to tell magicksave to write BMP.
This patch adds stub subclasses so that the savers know the exact format. It also improves PPM save.
By default, a dynamically loadable module is built for *magick (i.e.
`--with-magick=module`) when:
* ImageMagick or GraphicsMagick is found;
* GModule is supported (`gmodule_supported` pkg-config variable).
This can be overridden on the command line with:
* `--without-magick` - to disable *magick usage;
* `--with-magick[=yes]` - to restore the previous behavior;
* `--disable-modules` - to disable the build of dynamic modules.