Repaint region is wrong on Retina display with Qt5

With master, I am getting only the upper left-hand quadrant of a plot window painted.  Bisecting points to 
```
commit 252f4135799027f9ae887e8fd87e3af906187a22
Author: Antony Lee <anntzer.lee@gmail.com>
Date:   Thu Jul 27 22:32:35 2017 -0700

    Let QPaintEvent tell us what region to repaint.
```
This came in with #8951.