preformance issues with oxygen style and window decoration

[Maksim Orlovich]

> Well, the above will likely negate what I am about to say; but this thread
> reminded me that I've seen a hotspot/inefficiency in oxygen in some
> profiles, so I looked at it some more. In short, could you please try the
> attached patch? It's proof-of-concept only; it has not been reviewed
> by/discussed with Oxygen developers, but it'll hopefully say if it's worth
> addressing or not.

boemann has committed a proper patch for this (which conflicts with the
one I posted); though feedback on whether things got better would still be
welcome.