Cache QTextLayout objects with Qt/Mac

It seems that the internal Qt5 caching does not work well here,
especially in the case of ancient Hebrew documents.

(cherry picked from commit 5354c64b27)

src/frontends/qt4/GuiFontMetrics.cpp

@@ -58,7 +58,8 @@ namespace frontend {
 int cache_metrics_width_size = 1 << 19;
 int cache_metrics_breakat_size = 1 << 19;
 // Qt 5.x already has its own caching of QTextLayout objects
-#if (QT_VERSION < 0x050000)
+// but it does not seem to work well on MacOS X.
+#if (QT_VERSION < 0x050000) || defined(Q_OS_MAC)
 int cache_metrics_qtextlayout_size = 500;
 #else
 int cache_metrics_qtextlayout_size = 0;

status.22x

@@ -55,7 +55,10 @@ What's new
 - fixes to cursor positionning on mouse click for right-to-left text
   (bug 10569).
 
-- Fix metricsof full-width insets when paragraph endings are shown (bug #10537).
+- Fix metrics of full-width insets when paragraph endings are shown
+  (bug #10537).
+
+- Fix glacial performance with ancient Hebrew text on macOS.
 
 
 * INTERNALS