UI: Disable WA_PaintOnScreen for projectors

Removing this attribute fixes #3582 and allows projector windows to be resized
under KDE.

UI/window-projector.cpp

@@ -30,6 +30,11 @@ OBSProjector::OBSProjector(QWidget *widget, obs_source_t *source_, int monitor,
 	if (isAlwaysOnTop)
 		setWindowFlags(Qt::WindowStaysOnTopHint);
 
+#if defined(__linux__) || defined(__FreeBSD__) || defined(__DragonFly__)
+	// Prevents resizing of projector windows
+	setAttribute(Qt::WA_PaintOnScreen, false);
+#endif
+
 	type = type_;
 #ifdef __APPLE__
 	setWindowIcon(