Mesa drivers require access to up to date default drirc settings

Mesa stores its default application-specific settings and quirks inside /usr/share/drirc.d/00-mesa-defaults.conf. With the current Soldier runtime, Mesa drivers end up using an old or incomplete copy of that file which is included in the container and is missing many entries. Without those, a number of games end up being broken to various degrees. To name one example, Doom 3 BFG shows a black screen with a static white quad covering the top-right corner.

Ideally Mesa should be able to access the .conf file from the host system. If that's problematic, updating / improving the version of 00-mesa-defaults.conf included in Soldier should also work.