Sets the subpixel order for the font options object. The subpixel
order specifies the order of color elements within each pixel on
the display device when rendering with an antialiasing mode of
#CAIRO_ANTIALIAS_SUBPIXEL.
The following constants can be passed in the subpixel_order argument:
#CAIRO_SUBPIXEL_ORDER_DEFAULT-
Use the default subpixel order for for the target device.
#CAIRO_SUBPIXEL_ORDER_RGB-
Subpixel elements are arranged horizontally with red at the left.
#CAIRO_SUBPIXEL_ORDER_BGR-
Subpixel elements are arranged horizontally with blue at the left.
#CAIRO_SUBPIXEL_ORDER_VRGB-
Subpixel elements are arranged vertically with red at the top.
#CAIRO_SUBPIXEL_ORDER_VBGR-
Subpixel elements are arranged vertically with blue at the top.