Add mesa-next (latest mesa version)

With core-updates who-knows-how-far away and changes being proposed to a mesa package for better nvidia support, I thought we might as well add a nonguix version of mesa. For now this merely brings in the versions from core-updates, with a couple of minor changes (newer libdrm is called libdrm-next, use wayland-protocols-next from master, variable name for mesa is mesa-next). This is the latest version of mesa.

I then used it in place of mesa in the steam package, where I verified this version is picked up and works. I did not look at what should be done on the nvidia side with this change. Likely any changes needed in general for nvidia in nonguix (the whole libglvnd thing) will be done to the nonguix version of mesa, so we can consolidate that.

All feedback and testing welcome. The mesa module can be added easily (doesn't change anything else), while for steam I don't want to break nvidia so that requires input.