Set device pixel ratio for grim screenshot

Grim returns a bitmap with the physical size of the screen.
This needs to be scaled back to be displayed as underlay for Flameshot
on screens where logical size differs from the physical size (i.e. scale is active).

In order to do this, set the device pixel ratio, see
<https://doc.qt.io/qt-5/qpixmap.html#setDevicePixelRatio>.

Please note that this works only properly with integer scale.
`QScreen::devicePixelRatio()` rounds when fractional scale is active
on Wayland (because fractional scale is not yet supported by qtWayland).

Author: tobiolo

src/utils/screengrabber.cpp

@@ -37,6 +37,8 @@ void ScreenGrabber::generalGrimScreenshot(bool& ok, QPixmap& res)
     Process.start(program, arguments);
     if (Process.waitForFinished()) {
         res.loadFromData(Process.readAll());
+        QScreen* currentScreen = QGuiAppCurrentScreen().currentScreen();
+        res.setDevicePixelRatio(currentScreen->devicePixelRatio());
         ok = true;
     } else {
         ok = false;