winemac: High-res/retina mode fixes
This fixes two issues:
- since WineContentView became layer-backed in Wine 6.17, windows do not display correctly on a non-retina monitor when high-res/retina mode is enabled. (The needed downscaling was not being done).
- additionally on macOS 10.13 and earlier, the desired minification/downscaling filter was being ignored, causing poor image quality. Enabling rasterization for the layer seems to work around this and uses the correct filter.