Code review comment for lp:~mir-team/mir/unify-pointer-button

(0) MirPointerButton: Definitely a client ABI break now, so we must remember to bump to libmirclient9 ASAP (Alan is working on it but blocked by CI annoyances).

(1) I guess this isn't well tested yet because there's a logic error (should be "&="):
365 - buttons.erase(remove(begin(buttons), end(buttons), button));
366 + buttons |= ~button;