Meson-generated pkg-config files are seeded from the shared library
object, but that will use the Requires.private field for the library
dependencies.

Since Clutter exposes types from other libraries, the dependencies
should go into the Requires field, to avoid under-linking of dependent
projects. Additionally, per-backend pkg-config files should have their
own dependencies expressed into the Requires field, if they expose types
from those dependencies in the public API.