gtk3+wayland: dialogs work a lot better if a min size is set

instead of just an initial size, because we can set a size-request for the
minimum size and that's a reliable thing

Change-Id: I83916715cb9e3dceb6e88f3ca8fc86920677c026

vcl/source/window/btndlg.cxx

@@ -205,7 +205,7 @@ void ButtonDialog::ImplPosControls()
             nY += maCtrlSize.Height()+IMPL_SEP_BUTTON_Y;
     }
 
-    SetOutputSizePixel( aDlgSize );
+    SetMinOutputSizePixel(aDlgSize);
 
     mbFormat = false;
 }