Fixed 'missing glyph doesn't use fontconfig config substitutions' bug

XftFontMatch does display-specific font configuration (commit 528241a).
Nice. Unfortunately, when we switched from FcFontMatch, we also stopped
storing the post-Fc{Config,Default}Substitute FcPattern for future
lookups. The result is that if a glyph isn't found in the primary font,
secondary font lookups use the original FcPattern, not the configured
one. If you have custom fontconfig rules (like me), this can be
disappointing.

I basically just copied the guts out of XftFontMatch[1] and saved
the intermediate configured FcPattern. Could be related to the bug that
inspired commit 4242027.

[1]: https://cgit.freedesktop.org/xorg/lib/libXft/tree/src/xftfont.c

st.c

@@ -3373,16 +3373,32 @@ xgeommasktogravity(int mask)
 int
 xloadfont(Font *f, FcPattern *pattern)
 {
+	FcPattern *configured;
 	FcPattern *match;
 	FcResult result;
 	XGlyphInfo extents;
 	int wantattr, haveattr;
 
-	match = XftFontMatch(xw.dpy, xw.scr, pattern, &result);
-	if (!match)
+	/*
+	 * Manually configure instead of calling XftMatchFont
+	 * so that we can use the configured pattern for
+	 * "missing glyph" lookups.
+	 */
+	configured = FcPatternDuplicate(pattern);
+	if (!configured)
+		return 1;
+
+	FcConfigSubstitute(NULL, configured, FcMatchPattern);
+	XftDefaultSubstitute(xw.dpy, xw.scr, configured);
+
+	match = FcFontMatch(NULL, configured, &result);
+	if (!match) {
+		FcPatternDestroy(configured);
 		return 1;
+	}
 
 	if (!(f->match = XftFontOpenPattern(xw.dpy, match))) {
+		FcPatternDestroy(configured);
 		FcPatternDestroy(match);
 		return 1;
 	}
@@ -3414,7 +3430,7 @@ xloadfont(Font *f, FcPattern *pattern)
 		strlen(ascii_printable), &extents);
 
 	f->set = NULL;
-	f->pattern = FcPatternDuplicate(pattern);
+	f->pattern = configured;
 
 	f->ascent = f->match->ascent;
 	f->descent = f->match->descent;