Use XftFontMatch in place of FcFontMatch.

git am -s didn't like your patch:

From: Mark Edgar <medgar123@gmail.com>

XftFontMatch calls XftDefaultSubstitute which configures various match
properties according to the user's configured Xft defaults (xrdb) as well as
according to the current display and screen. Most importantly, the screen DPI
is computed [1]. Without this, st uses a "default" DPI of 75 [2].

[1]: https://cgit.freedesktop.org/xorg/lib/libXft/tree/src/xftdpy.c?id=libXft-2.3.2#n535
[2]: https://cgit.freedesktop.org/fontconfig/tree/src/fcdefault.c?id=2.11.1#n255

st.c

@@ -3279,7 +3279,7 @@ xloadfont(Font *f, FcPattern *pattern)
 	FcResult result;
 	XGlyphInfo extents;
 
-	match = FcFontMatch(NULL, pattern, &result);
+	match = XftFontMatch(xw.dpy, xw.scr, pattern, &result);
 	if (!match)
 		return 1;
 
@@ -3345,9 +3345,6 @@ xloadfonts(char *fontstr, double fontsize)
 		defaultfontsize = usedfontsize;
 	}
 
-	FcConfigSubstitute(0, pattern, FcMatchPattern);
-	FcDefaultSubstitute(pattern);
-
 	if (xloadfont(&dc.font, pattern))
 		die("st: can't open font %s\n", fontstr);