Revert "Use the raster rendering engine by default on OSX"

This reverts commit 22c5ee283d275d03e86250ed214ac91868b8de26.
The commit unfortunately caused low-quality on Retina displays.

Another idea would be to check if the display is retina and fallback to raster if that's not the case. I could not find a clean and simple way to do that. See also https://blog.qt.digia.com/blog/2013/04/25/retina-display-support-for-mac-os-ios-and-x11/