Make sure that drop shadows are deactivated in desktop mode

On OS X, making annotations in desktop mode then switching to board
mode and back could cause shadows to be drawn around every stroke, and
these persisted after erasing the strokes (though they did disappear
upon switching to board mode and back again).

src/desktop/UBDesktopAnnotationController.cpp

@@ -85,7 +85,7 @@ UBDesktopAnnotationController::UBDesktopAnnotationController(QObject *parent, UB
 #ifdef Q_OS_OSX
     mTransparentDrawingView->setAttribute(Qt::WA_MacNoShadow, true);
 #endif
-    mTransparentDrawingView->setWindowFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint | Qt::Window);
+    mTransparentDrawingView->setWindowFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint | Qt::Window | Qt::NoDropShadowWindowHint);
     mTransparentDrawingView->setCacheMode(QGraphicsView::CacheNone);
     mTransparentDrawingView->resize(UBApplication::desktop()->width(), UBApplication::desktop()->height());