Another go at fixing the jumpy scroll
The most recent problem was that we were setting _lastSetScroll whenever we
wrote to scrollTop (and ignoring the next scroll event which matched that
offset), but if there was no change to scrollTop, we wouldn't actually get a
scroll event, so would ignore some future scroll event instead.
Make sure that we only set _lastSetScroll if there's a change to scrollTop.
(Fixes https://github.com/vector-im/vector-web/issues/1162, more)
