Issue #6210: Subpixel font rendering system settings should only…

… apply to GIMP GUI not text layer rendering in image.

Reviewer note: this is the theoretical fix, but it won't work right now
because Cairo explicitly bypasses grayscale antialiasing when system set
subpixel one. Still let's push this first patch, but the issue will be
actually fixed when Cairo will merge my MR too:
https://gitlab.freedesktop.org/cairo/cairo/-/merge_requests/114

app/text/gimptextlayout.c

@@ -701,7 +701,7 @@ gimp_text_get_font_options (GimpText *text)
   cairo_font_options_t *options = cairo_font_options_create ();
 
   cairo_font_options_set_antialias (options, (text->antialias ?
-                                              CAIRO_ANTIALIAS_DEFAULT :
+                                              CAIRO_ANTIALIAS_GRAY :
                                               CAIRO_ANTIALIAS_NONE));
 
   switch (text->hint_style)