meson: Override dependencies to improve usage as a subproject

With this change, gdk-pixbuf can be consumed as a subproject without making any changes to the build files of a project. All you need to do is provide a wrap file with a [provide] section:

https://mesonbuild.com/Wrap-dependency-system-manual.html#provide-section

This is also necessary because otherwise projects need to hard-code the subproject name, which might be gdk-pixbuf when using wrap-git or gdk-pixbuf-2.42.10 when using wrap-file (to build from a release tarball). This can cause conflicts between different subprojects that consume gdk-pixbuf differently.

Other projects like glib, cairo, pango, etc already do this.