term: guess subpixel mode after initializing wayland window

At this point, we're not mapped, but we should have all the outputs
initialized. Which means we can at least guess which subpixel mode to
use.

If that turns out to be wrong, something we'll detect when we're
mapped, we'll just have to re-render.

terminal.c

@@ -842,6 +842,8 @@ term_init(const struct config *conf, struct fdm *fdm, struct wayland *wayl,
     if (!term_font_dpi_changed(term))
         goto err;
 
+    term->font_subpixel = get_font_subpixel(term);
+
     term_set_window_title(term, conf->title);
 
     /* Let the Wayland backend know we exist */