Read marker: not displayed when on a call event

What happened:

- I had a lot of call exchanges in a room between 2 users, so the room is filled with voip events.
I was in the bottom of the room and hit the "Jump to unread marker" and it jump to a place where no read marker appeared.

I was able to do this a lot of time and the invisible unread marker was still at the same place because the focus on the room was always the same.

When I opened the room on riot-web with the same user, the unread marker was on a "riotuser1 placed a video call" event, but this event was far before the one showed by riot-ios.

Reproduce the issue in an iOS room containing many VoIP call events, then use “Jump to unread marker” and compare the result with the same room in riot-web. Done means the unread marker is visible and points to the correct call event rather than an invisible or incorrectly positioned location.

Written by the indexing model from the issue text.