Overview window redraws every hidden column region row by row

One a large alignment and create many hidden columns (e.g. by hiding insertions). Open the overview window - it takes several seconds to load, and considerably longer than if there are no hidden columns. This happens (partly) because the implementation draws the overview row by row, and each time has to check again whether particular columns are hidden or not. A better implementation would be to do this once for a single row and create a hidden columns image to overlay onto the image without hidden columns (and similarly for hidden rows).