WIP: Update mesa to 20.0.8
Mesa 20.0 brings significant improvements in stability and correctness on our GPU. I've been running it for months on my phone and I think it should finally reach our users as well. It makes SuperTuxKart playable, allows us to drop workarounds in Animatch and fixes bunch of smaller issues happening in various GL apps.
Mesa 20.1 brings some regressions (for instance running neverball breaks the whole compositor), and Mesa 20.2 becomes much harder to package for amber; so I think 20.0.8 is a great candidate to be the last version to reach amber-phone before we switch to byzantium.
WIP because it needs newer libglvnd to reach amber-phone first (thankfully a straight rebuild from bullseye does its job there).