add a mailmap-file

This mailmap-file is based on the output of "git shorlog -se", and then taken duplicate entries and trying to figure out what the "preferred" or current email address is. Most of those were picked from Mesa's mailmap-file, which is likely to contain the contributors preference already. Some were based on best-effort guesswork.

This allows us to see up-to-date-ish email addresses for old contributions, which can be useful.