Make ``draw_idle`` method on ``FigureCanvasBase`` a no-op if not in interactive mode

For backends that do not have an event loop associated with them

(e.g. non-GUI backends) the semantics of `.FigureCanvasBase.draw_idle` are

not clear. Previously it was equivalent to a non-re-entrant

``fig.canvas.draw()`` call in all cases. Now it will be a no-op

unless Matplotlib is in "interactive mode" (as given by

`matplotlib.is_interactive`).

There are a number of derived properties in the Figure that we defer

computing until draw time (limits, ticks, the size of things in screen

space, optimized layout, etc). If you need to be sure that these

values are correct (with any backend) call ::

to force the Figure to fully render. This may break user code if it

is relying on `pyplot.draw` (which internally calls ``draw_idle``)

for the same effect in non-interactive mode.