frametimings: reuse previous frame timing in common case

frametimings: reuse previous frame timing in common case

Typically, there won't be any references on old frame timings except for
the most recent timing. So instead of discarding these and re-entering
gslice twice, just steal the old frame timing and reuse it.

https://bugzilla.gnome.org/show_bug.cgi?id=765592