noclear / background clear=False

[stuaxo]

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]
shoebot/shoebot#235
Suggested by @azbulutlu after we did a shoebot workshop a couple of years ago at LibreGraphicsMeeting.