video feedback preparation

add feedback and noFeedback commands, along with extra savePass and
materials to use them… unfortunately doesn’t work at the moment - at
least on this machine - something to do with format of render targets(?)
add mutator-commands class, initial gene animation

‘gene(name, min=0, max=1)’ (‘g’ for short) defines parameters that
animate within a given range. Best to use brackets around arguments,
and first “name” argument is not (yet) optional.