Screenshare option for video streams at tables, posters, etc

[yarikoptic]

I was 99% certain I had seen screen sharing available during one of the recent OHBM test-runs. But I do not see it available anywhere ATM. ~~Ideally there should be an option to make any video div to become larger modal one, so if it is a screen share of the participant -- people could actually see what is going on.~~ -- there is already a "full screen" mode in Controls (users just need to figure out to get there)

edit: some WiP which might be relevant - https://github.com/sparkletown/sparkle/pull/1469