Mesa: Allow to use custom drirc by apps

A custom drirc file can be considered as a nice workaround for issues related to Mesa. It works very well for native packages. However, it is basically ignored by flatpak. First of all, flatpak packages don't have access to the /etc/drirc file from host. Moreover, most games are completely sandboxed when it comes to the filesystem, so they can't read ~/.drirc neither.

There should be an easy way to use a custom drirc file in flatpak packages, so maintainers could fix theirs games for specific drivers. Thanks to this patch, they will be able to modify the Mesa configuration in several places in /app/.

Closes #1079

Related to #1071 (comment 375480403)