The redraw flag starts out as true, then is immediately set to false on
draw.  If labels are enabled, drawPie calls drawLabels, which calls
drawLabel for each label.  Any label that can't fit sets redraw back to
true, so the whole process can repeat.

This isn't the most obvious mechanism, and forces one to remember to do
things like setting redraw back to true after drawing, so the plot can
redraw itself on resize or when setting new data.

Instead we now have drawPie return true when it drew successfully, and
false otherwise, which the same happening in drawLabels and drawLabel.
Instead of checking the flag, we now just check the return value.

This has the added benefit of slightly improving performance in the case
where several redraws are necessary, since it now short-circuits out of
the draw loop as soon as one label fails to fit.