Code review comment for lp:~alan-griffiths/mir/fix-1526658

I did initially agree with your "hypothetical parallel installation that requires mesa2" assertion. However that's not true if the older Mir installation uses libmirclient9. All recent Mir releases use libmirclient9, which means even old clients will start using the new libmirclient9 immediately and only need mir-client-platform-mesa4 to work.