add feedback demo

also removed “need to work out proper place for feedback” comments that
had gone stale.
add mutate function, modify demos
Backgrounds work again. Cleanup demos

At some point in getting video feedback to work I thought it might help
using an RGB format for render buffers, but RGBA is fine, allowing the
background command to work as before. Due to being a separate div,
background is not incorporated into the feedback (which could be a
useful way of seeding it).
Some adjustments to syntax in demos as indicated by @davidedc in
comments.
Also using mutate function in doOnce in some demos, to be applied
interactively.