Fix detection of DPI changes in builds without Xft

* src/xsettings.c (apply_xft_settings) [!HAVE_XFT]: Support
XSETTINGS changes in non-XFT builds.  (Bug#43128)

Copyright-paperwork-exempt: yes

src/xsettings.c

@@ -804,16 +804,28 @@ static void
 apply_xft_settings (Display_Info *dpyinfo,
                     struct xsettings *settings)
 {
-#ifdef HAVE_XFT
+#if defined USE_CAIRO || defined HAVE_XFT
   FcPattern *pat;
   struct xsettings oldsettings;
   bool changed = false;
+#ifndef HAVE_XFT
+  cairo_font_options_t *options;
+#endif
+
 
   memset (&oldsettings, 0, sizeof (oldsettings));
   pat = FcPatternCreate ();
+#ifdef HAVE_XFT
   XftDefaultSubstitute (dpyinfo->display,
                         XScreenNumberOfScreen (dpyinfo->screen),
                         pat);
+#else
+  FcConfigSubstitute (NULL, pat, FcMatchPattern);
+  options = cairo_font_options_create ();
+  cairo_ft_font_options_substitute (options, pat);
+  cairo_font_options_destroy (options);
+  FcDefaultSubstitute (pat);
+#endif
   FcPatternGetBool (pat, FC_ANTIALIAS, 0, &oldsettings.aa);
   FcPatternGetBool (pat, FC_HINTING, 0, &oldsettings.hinting);
 #ifdef FC_HINT_STYLE
@@ -912,8 +924,11 @@ apply_xft_settings (Display_Info *dpyinfo,
 		     - sizeof "%f")
       };
       char buf[sizeof format + d_formats * d_growth + lf_formats * lf_growth];
-
+#ifdef HAVE_XFT
       XftDefaultSet (dpyinfo->display, pat);
+#else
+      FcPatternDestroy (pat);
+#endif
       store_config_changed_event (Qfont_render,
 				  XCAR (dpyinfo->name_list_element));
       Vxft_settings