QFontMetrics::xHeight() is not always the ascent of 'x'

Use the real ascent of glyph 'x' as LyX 2.3.x did.
It can make a difference at least on Windows with many fonts.

Fixes bug #13100.

(cherry picked from commit 194b53ef9d)

src/frontends/qt/GuiFontMetrics.cpp

@@ -114,7 +114,7 @@ inline QChar const ucs4_to_qchar(char_type const ucs4)
 
 
 GuiFontMetrics::GuiFontMetrics(QFont const & font)
-	: font_(font), metrics_(font, 0), xheight_(metrics_.xHeight()),
+	: font_(font), metrics_(font, 0), xheight_(-metrics_.boundingRect('x').top()),
 	  strwidth_cache_(strwidth_cache_max_cost),
 	  breakstr_cache_(breakstr_cache_max_cost),
 	  qtextlayout_cache_(qtextlayout_cache_max_size)
@@ -163,6 +163,7 @@ int GuiFontMetrics::em() const
 
 int GuiFontMetrics::xHeight() const
 {
+	// This is not alays true on Windows (see #13100).
 	// LATTEST(metrics_.xHeight() == ascent('x'));
 	return xheight_;
 }

status.24x

@@ -35,6 +35,8 @@ What's new
   These two settings are mutually exclusive by default and we signal
   that within UI now. Workarounds can be found our and hyperref manual.
 
+- Fix display of superscripts on Windows (bug 13100).
+
 
 * DOCUMENTATION AND LOCALIZATION