Comment 4 for bug 1530697

I haven't debugged yet, but I think this may be because of my hidpi display:

http://blog.qt.io/blog/2013/04/25/retina-display-support-for-mac-os-ios-and-x11/

"""
QGlWidget

OpenGL operates in device pixel space. For example, the width and height passed to glViewport should be in device pixels. QGLWidget::resizeGL() gives the width and height in device pixels.

However, QGLWidget::width() is really QWidget::width() which returns a value in device-independent pixels. Resolve it by multiplying with widget->windowHandle()->devicePixelRatio() if needed.
"""

Now that devicePIxelRatio is exposed by Qt we need to scale the widget width/height we pass to the shaders by it.