[OS X] Support for HiDPI resolutions
Changes needed for supporting HiDPI resolutions:
- All hardcoded pixel sizes in runtime images and drawing offsets must be scaled to device pixels (widgets,
- A Doomsday Script variable must be provided for scripts to check the current DPI factor.
- UI style must apply the DPI factor in rules, font sizes, and fixed-size images.
- Mouse coordinates must be translated to device coordinates.
- The GUI Shell's log history must be scaled manually since it is drawn in an offscreen buffer.