[Freeciv-Dev] (PR#2982) iso-view FOW drawing bug

["Jason Short via RT" <rt@xxxxxxxxxxxxxx>]

Sometimes when drawing is done in iso-view, the fog-of-war is not updated.

Typically this occurrs when a unit enters a fogged area.  Clearly the
area is no longer fogged, but this is not immediately updated.  Instead
the unit itself is drawn.  This means parts of the surrounding tiles are
drawn, and these are unfogged.  The end result is that the area is still
fogged, but the box around the unit is unfogged.

This is difficult to reproduce.  Occasionally it happens with the GTK
client at the very beginning of a game.  With a lot of buffering of the
drawing, it seems it can happen all the time.

jason