doc/QtPlayer: set WS_CLIPCHILDREN on our window

So the display module doesn't have to do it in its own thread.
It doesn't seem to be on by default in Qt and there's no direct API to set this.

doc/libvlc/QtPlayer/player.cpp

@@ -149,6 +149,9 @@ void Mwindow::openFile() {
 #elif defined(Q_OS_UNIX)
     libvlc_media_player_set_xwindow(vlcPlayer, videoWidget->winId());
 #elif defined(Q_OS_WIN)
+    LONG style = GetWindowLong( videoWidget->winId(), GWL_STYLE );
+    if( !(style & WS_CLIPCHILDREN) )
+        SetWindowLong( videoWidget->winId(), GWL_STYLE, style | WS_CLIPCHILDREN );
     libvlc_media_player_set_hwnd(vlcPlayer, (HWND)videoWidget->winId());
 #endif