add libva as subproject
In gst-plugins-bad!1379 (comment 552926) @dabrain34 proposed to add libva as a subproject so old distros could compile gstva, in bad, and gstreamer-vaapi.
It's a bit complex, mainly because the libva version is tightly coupled with the used driver, and a driver might need be added as subproject too.
But also libva needs directory layout changes to support uninstalled fallback: https://github.com/intel/libva/pull/433