Update Still Image requirements in AV2 requirements proposal

[leo-barnes]

Compared to the other sections in the AV2 requirements proposal, the section 2.9 on still images is relatively vague. We should identify the use-cases we want to support and update it with more specific requirements for those use-cases. Using section 2.1 as a template might be a good idea.

Once we have identified use-cases and requirements, we also need to update section 3.

[leo-barnes]

I'll start a discussion on the list with the slides I showed at the meeting 2020-08-06 as a starting point.