Instead of hardcoding gtk-xft-antialias, use SPI_GETFONTSMOOTHING to
determine whether antialiasing is enabled.
Make gtk-xft-rgba query more complex - try to determine display
orientation, then use that to rotate subpixel structure. This
won't help with monitors that have naturally vertical subpixels,
but should improve things for monitors that are rotated (as long
as Windows display settings are adjusted accordingly).
Partially fixes#1774
