Change numBytes() to byteCount()

numBytes() is obsolete in Qt 4.8 and removed in Qt5.  byteCount() is a
drop in replacement that works in 4.8 and 5.

src/Gui/BitmapFactory.cpp

@@ -589,7 +589,7 @@ void BitmapFactoryInst::convert(const QImage& p, SoSFImage& img) const
     size[0] = p.width();
     size[1] = p.height();
 
-    int buffersize = p.numBytes();
+    int buffersize = p.byteCount();
     int numcomponents = 0;
     QVector<QRgb> table = p.colorTable();
     if (!table.isEmpty()) {