Implement maxCharWidth() for DirectWrite engine

This is used to create the bounding box in QFontEngine::properties(), which
in turn is used for the FontBBox when generating PDFs. The result was that
the bounding width in the output was 0 and Adobe Reader complained that the
PDF was malformed. We could implement the proper bounding rect in properties()
at some point, instead of assuming an origin at x = 0 for instance. The
metrics for that are in the head table. But for silencing the warning in
Reader, just implementing the maxCharWidth() function is sufficient.

[ChangeLog][Windows][PDF] Fixed a bug in PDF output when using high-dpi
scaling which was causing the display of warnings when opening the
file in Adobe Reader.

Task-number: QTBUG-58954
Change-Id: I2540571863d4dd0f85af533b591f75dad3f0d75b
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/platformsupport/fontdatabases/windows/qwindowsfontenginedirectwrite.cpp

@@ -355,6 +355,13 @@ void QWindowsFontEngineDirectWrite::collectMetrics()
         m_faceId.filename = QFile::encodeName(filenameFromFontFile(fontFile));
         fontFile->Release();
     }
+
+    QByteArray table = getSfntTable(MAKE_TAG('h', 'h', 'e', 'a'));
+    const int advanceWidthMaxLocation = 10;
+    if (table.size() >= advanceWidthMaxLocation + sizeof(quint16)) {
+        quint16 advanceWidthMax = qFromBigEndian<quint16>(table.constData() + advanceWidthMaxLocation);
+        m_maxAdvanceWidth = DESIGN_TO_LOGICAL(advanceWidthMax);
+    }
 }
 
 QFixed QWindowsFontEngineDirectWrite::underlinePosition() const
@@ -607,8 +614,9 @@ QFixed QWindowsFontEngineDirectWrite::xHeight() const
 
 qreal QWindowsFontEngineDirectWrite::maxCharWidth() const
 {
-    // ###
-    return 0;
+    return fontDef.styleStrategy & QFont::ForceIntegerMetrics
+           ? m_maxAdvanceWidth.round().toReal()
+           : m_maxAdvanceWidth.toReal();
 }
 
 QImage QWindowsFontEngineDirectWrite::alphaMapForGlyph(glyph_t glyph, QFixed subPixelPosition, const QTransform &t)

src/platformsupport/fontdatabases/windows/qwindowsfontenginedirectwrite_p.h

@@ -143,6 +143,7 @@ private:
     QFixed m_descent;
     QFixed m_xHeight;
     QFixed m_lineGap;
+    QFixed m_maxAdvanceWidth;
     FaceId m_faceId;
     QString m_uniqueFamilyName;
 };