On high-resolution Retina displays, the OS X window backing store has twice the
pixels as Wine's window backing store.  So, our images get scaled up.  Core
Graphics had been interpolating/smoothing the image, which resulted in
fuzziness.  This tells it not to do that.

I had assumed this wouldn't be necessary since we pass FALSE for the
shouldInterpolate parameter of CGImageCreate() when we create the images.
Apparently, that's not sufficient.