Code review comment for lp:~mir-team/mir/cursor-spike-phase-1-resubmit

#1

> Probably for the themed cursors, it didnt seem worth defining the ctor in this case since its only called
> once, but I decided there is no harm and changed it to define a ctor so we can just return the same instance.
>
> Strictly this code is probably less performant ;) as there is no runtime difference (only called once) and the
> non defaulted ctor increases binary size. Still I dont think there is any harm anyway.

I'm not so concerned about the performance either here. It seems in the superseded review the make_shared() was in lookup_cursor, and in this one its in the constructor of BuiltinCursorRepository, so this is okay by me.