Reduce overlays icon size
Measure latency between peers and add feedback
Previews will be grey when latency is unknown (not measured yet), green when it's good (60ms or less), blue when medium (between 60 and 120), orange when it's becoming high (between 120 and 250) and red when latency is too high to have a good call (250ms or more)
To reduce confusion, slected preview will now have a reduced opacity instead of the red border
Fix #66