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/InputHintWidget.cpp

@@ -136,15 +136,12 @@ QPixmap Gui::InputHintWidget::generateKeyIcon(const InputHint::UserInput key, co
     const QFontMetrics fm(font);
     const QString text = inputRepresentation(key);
     const QRect textBoundingRect = fm.tightBoundingRect(text);
-    const qreal dpr = BitmapFactoryInst::getMaximumDPR();
 
     const int symbolWidth = std::max(textBoundingRect.width() + padding * 2, iconSymbolHeight);
 
     const QRect keyRect(margin, margin, symbolWidth, iconSymbolHeight);
 
-    QPixmap pixmap((symbolWidth + margin * 2) * dpr, height * dpr);
-    pixmap.fill(Qt::transparent);
-    pixmap.setDevicePixelRatio(dpr);
+    QPixmap pixmap = BitmapFactory().empty({ symbolWidth + margin * 2, height });
 
     QPainter painter(&pixmap);
     painter.setRenderHint(QPainter::Antialiasing);