Fixes #85850.

Motivation for this change

I built libva-minimal, libva (covers mesa and libGL as well), and libva-utils.

Update:

• Tested: VA using mpv with my old HD 6950 (r600): Worked fine
• Built: intel-media-driver vaapiIntel vaapiVdpau libvdpau-va-gl

Testing was only done via vainfo:

libva info: VA-API version 1.7.0
libva info: Trying to open /run/opengl-driver/lib/dri/r600_drv_video.so
libva info: Found init function __vaDriverInit_1_6
libva info: va_openDriver() returns 0
vainfo: VA-API version: 1.7 (libva 2.7.1)
vainfo: Driver version: Mesa Gallium driver 20.0.2 for AMD CAYMAN (DRM 2.50.0 / 5.4.31, LLVM 9.0.1)
vainfo: Supported profile and entrypoints
      VAProfileMPEG2Simple            :	VAEntrypointVLD
      VAProfileMPEG2Main              :	VAEntrypointVLD
      VAProfileVC1Simple              :	VAEntrypointVLD
      VAProfileVC1Main                :	VAEntrypointVLD
      VAProfileVC1Advanced            :	VAEntrypointVLD
      VAProfileH264ConstrainedBaseline:	VAEntrypointVLD
      VAProfileH264Main               :	VAEntrypointVLD
      VAProfileH264High               :	VAEntrypointVLD
      VAProfileNone                   :	VAEntrypointVideoProc

Rebuilding my whole system with this change would take way too long, but I could consider rebuilding a subset (focusing on important direct dependencies) if that would help.
Update: See above, I guess this update should be fine (or at least mostly).

Note: I added myself as maintainer as there wasn't one and I noticed both packages could use some other changes/improvements as well, but I'll do that in a separate PR to avoid too many changes at once.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.