If interactive mode is on using the model where every time any artist is
invalidated/marked as stale a `draw_idle` is triggered and the user is
using a non-Agg based backend, saving a png will result in a draw_idle
call triggered from inside the __init__ method of `FigureCanvasBase`
which then fails because the full object has not been set up (this is
not a problem using the IPython hooks because the stale state is only
checked once when all user code has completed executing).

closes matplotlib#5094

This is not part of the state of the figure that will trigger a
re-drawing.

If the user resets the canvas, it is their responsibility to schedule
the redraw.

Set the `_is_saving` flag as early as possible in `print_figure`
to prevent aggressive auto-drawing during the same process.

Random thrashing to prevent the intermittent failures