Fix a redraw glitch when hiding a widget.

When hiding the grid in the game_load dialog it wasn't redrawn properly,
this fixes it. (The changes uncovering this problem will be committed
later.)

changelog

@@ -9,6 +9,7 @@ Version 1.7.10+svn:
    * Scrollbar containers now use the button super class
    * Allow a gui2 timer delete itself in its callback
    * Fix various bugs when a unit has more MP than its maximum
+   * Fix not redrawing a grid when set to hidden
 
 Version 1.7.10-1.8beta3:
  * Campaigns:

src/gui/widgets/widget.cpp

@@ -182,7 +182,7 @@ void twidget::populate_dirty_list(twindow& caller,
 {
 	assert(call_stack.empty() || call_stack.back() != this);
 
-	if(visible_ != twidget::VISIBLE) {
+	if(visible_ == twidget::INVISIBLE) {
 		return;
 	}