Subgroup for GStreamer maintained meson ports

I think we should add a meson-ports subgroup where we maintain forks of various project with Meson ports for gst-build purposes.

This would collect various ports that are now maintained on personal or company github repos in one location where it can be maintained by all interested GStreamer developers.

This would not preclude any efforts to upstream such ports of course, or to upload them to Meson's wrapdb.

My primary goal for now is to move various things which we depend on now away from company repos, in particular: libffi, zlib, and gl-headers. (libnice is probably ready soon enough to be submitted directly to upstream.)