Future proofing, make sure we get the screen this window is in.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@25267 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/apps/mediaplayer/MainWin.cpp

@@ -660,7 +660,7 @@ void
 MainWin::WindowActivated(bool active)
 {
 	if (active) {
-		BScreen screen;
+		BScreen screen(this);
 		BRect screenFrame = screen.Frame();
 		BRect frame = Frame();
 		float diffX = 0.0;