Don't return a visual glyph for ignorable characters

When Harfbuzz returns zero glyphs, this will typically be because
the character is ignored. We currently have no way to differentiate
this from actual error cases, so to avoid return the error glyph
for cases which are not really errors, we set the dontPrint flag
for this.

When we add a way to differentiate between error cases and non-error
cases later, we should revert this for the actual errors.

Pick-to: 6.4
Task-number: QTBUG-108799
Change-Id: I2a910d951b71c705fb4dd761fcfe3a942b3afa7c
Reviewed-by: Tor Arne Vestbø <[email protected]>

src/gui/text/qtextengine.cpp

@@ -1528,6 +1528,7 @@ void QTextEngine::shapeText(int item) const
         QGlyphLayout g = availableGlyphs(&si);
         g.glyphs[0] = 0;
         g.attributes[0].clusterStart = true;
+        g.attributes[0].dontPrint = true;
 
         ushort *log_clusters = logClusters(&si);
         for (int i = 0; i < itemLength; ++i)

tests/auto/gui/text/qglyphrun/tst_qglyphrun.cpp

@@ -616,9 +616,7 @@ void tst_QGlyphRun::defaultIgnorables()
     layout.endLayout();
 
     QList<QGlyphRun> runs = layout.glyphRuns();
-    QCOMPARE(runs.size(), 1);
-    QCOMPARE(runs.at(0).glyphIndexes().size(), 1);
-    QCOMPARE(runs.at(0).glyphIndexes()[0], uint(0));
+    QCOMPARE(runs.size(), 0);
 }
 
 void tst_QGlyphRun::stringIndexes()