winemac: Prevent interpolation of the window surface image when it's blitted to the actual window.
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.