The pre-release version I tried to use to get the blend2d changes has a
problem that causes builds to fail when using the vcpkg executable
included in the GitHub Actions images. See here:
https://github.com/microsoft/vcpkg/pull/30396#issuecomment-3452587376
So I'll restore the blend2d changes as an overlay port in a separate
commit.
This is basically the addition of the vcpkg.json manifest and
vcpkg-configuration.json, which are both pretty minimal. In practice,
projects will probably copy those and add overlays.