BUG : Qt repaint workaround on windows - #2844

This works and should be merged. What happens under windows is that the update events are posted, but the painting happens quite unregular, sometimes only every 20th update. Don't know the reason why.

One remark: I think also non-windows should use repaint.
Reason: The draw() method should guarantee an repaint, at the moment
it has the behavior i except from draw_idle().

This should be merged before the next release.

@Tillsten That is the plan. It is waiting on some one else but me to push the big green button.

@tacaswell Any comment to the point i made?

As discussed in #2559 , I do not want to go against the suggestions of the QT docs with out a very good reason, in this case, the reason is that windows is a broken platform. The change makes no difference on linux (and I suspect mac as no mac-users have complained about this and there are a few devs who I know use both mac and the QT backend) so there is no reason to expose the other systems to the possible consequences of miss-using the gui toolkit. We should in general not be fighting the event-loop.

The behaviour I expect from draw_idle is that the mpl-level draw is only called when the system is idle. In this case, canvas.draw is hit every time draw is called.

Sorry to bring up this PR again, but i think this should go at least to changes.txt.

@Tillsten Yeah, you are right.

@Tillsten I have completely come around on the need for this on windows, sorry for the push back.

https://stackoverflow.com/questions/23167424/matplotlib-button-color-updates-only-after-moving-mouse?noredirect=1#comment35491004_23167424