- on iOS safari and chrome, in case we show eg a spinner until we get the videoConferenceJoined event, all `clientResize` are done with size 0 for width/height
- on iOS we never get a `clientResize` call with correct values, except if we force a call by eg opening/closing the chat window
audio-only,lastn: move audio-only and last N handling to standalone features
This refactors all handling of audio-only and last N to 2 features in preparation
for "low bandwidth mode".
The main motivation to do this is that lastN is a "global" setting so it helps
to have all processing for it in a single place.