See issue #5228. Currently, WL_OUTPUT_SUBPIXEL_NONE is ignored and
CAIRO_ANTIALIAS_SUBPIXEL is still set. This commit checks if subpixel is
set to none and if so, calls set_antialias with CAIRO_ANTIALIAS_GRAY.
This mirrors the functionality in Mako's
[PR261](https://github.com/emersion/mako/pull/261)