Work-around a Qt issue when starting in minimal mode

Close #3573
How can the size of a widget be bigger than the size of its parent window? Really weird Qt bug
(cherry picked from commit 8ca53bfd5abb38e5b4d4a8b89ce523e0f4d4aad1)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/main_interface.hpp

@@ -209,7 +209,10 @@ private slots:
 
     void resizeStack( int w, int h ) {
         if( !isFullScreen() && !isMaximized() )
-            resize( size() - stackCentralW->size() + QSize( w, h ) );
+            if( i_visualmode == 1 ) resize( w, h ); /* Oh yes, it shouldn't
+                                   be possible that size() - stackCentralW->size() < 0
+                                   since stackCentralW is contained in the QMW... */
+            else resize( size() - stackCentralW->size() + QSize( w, h ) );
         debug(); }