mailmap: misc cleanups

What does this MR do and why?

While looking into something else, I noticed there were a few stale and/or missing mailmap entries for Collabora employees. This is an attempt to fix them all.

@tomeu @bbeckett @gerddie @nanokatze @shadeslayer: These commits modify how you appear. Would you mind taking a look if you're OK with those changes? If I messed anything up, please let me know and I'll fix it!