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.

lib/unicodesymbols

@@ -2332,10 +2332,10 @@
 0x2224 ""                         "" "force!=utf8x" "\\nmid" "" # DOES NOT DIVIDE # or "\\notdivides" "mathabx"
 0x2225 ""                         "" "force!=utf8x" "\\parallel" "" # PARALLEL TO
 0x2226 ""                         "" "force!=utf8x" "\\nparallel" "amssymb" # NOT PARALLEL TO
-0x2227 ""                         "" "force!=utf8x" "\\bigwedge" "" # LOGICAL AND
-0x2228 ""                         "" "force!=utf8x" "\\bigvee" "" # LOGICAL OR
-0x2229 ""                         "" "force!=utf8x" "\\bigcap" # INTERSECTION
-0x222a ""                         "" "force!=utf8x" "\\bigcup" "" # UNION
+0x2227 ""                         "" "force!=utf8x" "\\wedge" "" # LOGICAL AND
+0x2228 ""                         "" "force!=utf8x" "\\vee" "" # LOGICAL OR
+0x2229 ""                         "" "force!=utf8x" "\\cap" # INTERSECTION
+0x222a ""                         "" "force!=utf8x" "\\cup" "" # UNION
 0x222b ""                         "" "force!=utf8x" "\\int" "esint" # INTEGRAL
 0x222c ""                         "" "force!=utf8x" "\\iint" "esint|amsmath" # DOUBLE INTEGRAL
 0x222d ""                         "" "force!=utf8x" "\\iiint" "esint|amsmath" # TRIPLE INTEGRAL

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)
@@ -163,7 +163,8 @@ int GuiFontMetrics::em() const
 
 int GuiFontMetrics::xHeight() const
 {
-//	LATTEST(metrics_.xHeight() == ascent('x'));
+	// This is not alays true on Windows (see #13100).
+	// LATTEST(metrics_.xHeight() == ascent('x'));
 	return xheight_;
 }