Bug#992665: json-glib: Update to 1.6.4

[Simon McVittie]

On Sat, 21 Aug 2021 at 20:01:35 -0400, Jeremy Bicha wrote:
> GNOME has released json-glib 1.6.4 but the documentation has been
> switched to gi-docgen. So we should wait for gi-docgen to be available
> in Debian before packaging the update.

gi-docgen is not yet stable, and the upstream developer recommends
vendoring it (as a Meson subproject) for now. When gi-docgen gets out
of NEW, I don't intend to move it from experimental to unstable until
its upstream developer is willing to support it being used un-vendored.

A vendored or un-vendored gi-docgen needs some patches and some
Files-Excluded for Debian use: see the gtk4 and pango1.0 packages for
examples.

    smcv