[Freeciv-Dev] (PR#7267) buffer/flush the overview

["Jason Short" <jdorje@xxxxxxxxxxxxxxxxxxxxx>]

<URL: http://rt.freeciv.org/Ticket/Display.html?id=7267 >

With the new-ish flush code for the mapview, some operations like 
end-game revealing of the map are much faster.

But these operations are still quite slow and ugly for the overview. 
This should have some form of flush code as well.

jason