noclear / background clear=False

I'm implementing a feature[1] in shoebot to allow the user to preserve what was drawn in the previous frame.

I think the API will look like this:

noclear() 
background(clear=False)

Although maybe something like this could be an alternative:
background(preserve=True)

It would be good to keep the nodebox1 implementations in sync, let me know what you think.

[1]
https://github.com/shoebot/shoebot/issues/235 Suggested by @azbulutlu after we did a shoebot workshop a couple of years ago at LibreGraphicsMeeting.