mesa: 20.3.4 -> 20.3.5

[primeos]

Motivation for this change

Yay :) (it's been almost two months since 20.3.4 which was a bit concerning since stable releases are normally tagged every two weeks).
The release notes aren't up yet: https://docs.mesa3d.org/relnotes/20.3.5.html

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.

[SuperSandro2000]

@ofborg eval

[vcunat]

Merged but immediately bumped to 21.0.1? 😄

[primeos]

Yeah, IIRC I didn't check my update feeds between opening and merging this PR so I wasn't aware that 21.0.1 was already released in the meantime but I'd've likely merged this PR regardless (or should I try to avoid that in the future? AFAIK it has both advantages and drawbacks). Anyway, I'm glad that 21.0.1 is finally out :)

[vcunat]

Not a problem; I just found found it funny to see those two merges so close together.