Fix read marker visibility for grouped events
The recent "groupers" which extracted out timeline grouping logic forgot to
pass through the last event state for read marker computation. This causes the
read marker to become visible when e.g. returning to room if it was last placed
inside a grouped set of events (currently room creation and membership events).
Regressed by https://github.com/matrix-org/matrix-react-sdk/pull/4059
Related to https://github.com/vector-im/riot-web/issues/12338
