Code review comment for lp:~mir-team/mir/expose-debug-buffer-id-to-client

For libmirclient we are providing stability, so a Mir update doesn't break mesa/XMir etc. Just mark functions as deprecated and we'll remove them in one big change (libmirclient1 -> libmirclient2). If you really want to get rid of a function now, then you have to bump ABI but this will probably be more work overall than just leaving the old function there.