p5: retain state from draw-to-draw

[sbinet]

in Processing and in p5js, the state of the display is retained from iteration to iteration.

right now, in p5, we don't (so, for example, in the mouse-pressed example, the circle around the mouse follows the mouse but doesn't leave a trail of circles across its path, as the p5js example does.)

that's actually a feature of Gio.

to "simulate" the same behaviour, we could have an image.Image field that is drawn to at the end of each draw cycle.
at cycle+1, we draw that last image.Image before invoking the user's draw function.