Release font engine refcount when done using it in QTextEngine

5f2b6dd2a50275bc05ae5d7e9dd8902d6d49d9df increased refcounts for
font engines in QTextEngine cache, we need to decrease them when
the QTextEngine is deallocated.

Task-number: QTBUG-21222
Reviewed-by: Eskil
(cherry picked from commit 8935a84e18804c7ff4b7336e3cfdf1cd558eaf1c)

Change-Id: I079c903d60e1bbcf78f555f8044dde69af82cf0e
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>

src/gui/text/qtextengine.cpp

@@ -1164,6 +1164,7 @@ QTextEngine::~QTextEngine()
     if (!stackEngine)
         delete layoutData;
     delete specialData;
+    resetFontEngineCache();
 }
 
 const HB_CharAttributes *QTextEngine::attributes() const
@@ -1224,6 +1225,13 @@ static inline void releaseCachedFontEngine(QFontEngine *fontEngine)
     }
 }
 
+void QTextEngine::resetFontEngineCache()
+{
+    releaseCachedFontEngine(feCache.prevFontEngine);
+    releaseCachedFontEngine(feCache.prevScaledFontEngine);
+    feCache.reset();
+}
+
 void QTextEngine::invalidate()
 {
     freeMemory();
@@ -1232,9 +1240,7 @@ void QTextEngine::invalidate()
     if (specialData)
         specialData->resolvedFormatIndices.clear();
 
-    releaseCachedFontEngine(feCache.prevFontEngine);
-    releaseCachedFontEngine(feCache.prevScaledFontEngine);
-    feCache.reset();
+    resetFontEngineCache();
 }
 
 void QTextEngine::clearLineData()

src/gui/text/qtextengine_p.h

@@ -631,6 +631,7 @@ public:
     int lineNumberForTextPosition(int pos);
     int positionAfterVisualMovement(int oldPos, QTextCursor::MoveOperation op);
     void insertionPointsForLine(int lineNum, QVector<int> &insertionPoints);
+    void resetFontEngineCache();
 
 private:
     void setBoundary(int strPos) const;

src/gui/text/qtextlayout.cpp

@@ -370,7 +370,7 @@ QTextLayout::~QTextLayout()
 void QTextLayout::setFont(const QFont &font)
 {
     d->fnt = font;
-    d->feCache.reset();
+    d->resetFontEngineCache();
 }
 
 /*!
@@ -511,7 +511,7 @@ void QTextLayout::setAdditionalFormats(const QList<FormatRange> &formatList)
     }
     if (d->block.docHandle())
         d->block.docHandle()->documentChange(d->block.position(), d->block.length());
-    d->feCache.reset();
+    d->resetFontEngineCache();
 }
 
 /*!