Gui: Add BitmapFactory::empty method

This adds empty(QSize) method to bitmap factory that creates empty
pixmap. This may seem useless, but after creating bitmap one needs to
remember to clear it and to properly set DPR - BitmapFactory will take
care of it.

src/Gui/BitmapFactory.cpp

@@ -521,6 +521,17 @@ QPixmap BitmapFactoryInst::disabled(const QPixmap& p) const
     return QApplication::style()->generatedIconPixmap(QIcon::Disabled, p, &opt);
 }
 
+QPixmap BitmapFactoryInst::empty(QSize size) const
+{
+    qreal dpr = getMaximumDPR();
+
+    QPixmap res(size * dpr);
+    res.fill(Qt::transparent);
+    res.setDevicePixelRatio(dpr);
+
+    return res;
+}
+
 void BitmapFactoryInst::convert(const QImage& p, SoSFImage& img) const
 {
     SbVec2s size;