Fix framebuffer size on fractional scaling display.

src/citra_qt/bootmanager.cpp

@@ -266,7 +266,7 @@ QByteArray GRenderWindow::saveGeometry() {
 }
 
 qreal GRenderWindow::windowPixelRatio() const {
-    return devicePixelRatio();
+    return devicePixelRatioF();
 }
 
 std::pair<u32, u32> GRenderWindow::ScaleTouch(const QPointF pos) const {