mapreveal: keep all revealed zones by the gameclient

Only extend with missing unrevealed zones instead of overwriting
the entire map reveal logic of the game. This is required to not
break custom servers/clients that are shipping their own zones.

modules/mapreveal.lua

@@ -215,10 +215,20 @@ pfUI:RegisterModule("mapreveal", "vanilla:tbc", function ()
         end
       end
     end
-    for i = textureCount + 1, NUM_WORLDMAP_OVERLAYS do
-      _G[string.format("%s%s","WorldMapOverlay",i)]:Hide()
-    end
   end
 
-  hooksecurefunc("WorldMapFrame_Update", pfWorldMapFrame_Update, true)
+  -- hook map reveal functions before and after the actual call
+  local pfHookWorldMapFrame_Update = _G.WorldMapFrame_Update
+  _G.WorldMapFrame_Update = function(self)
+    -- hide all previously set textures
+    for i = 1, NUM_WORLDMAP_OVERLAYS do
+      _G[string.format("%s%s","WorldMapOverlay",i)]:Hide()
+    end
+
+    -- let the game put its explored tiles on the map
+    pfHookWorldMapFrame_Update(self)
+
+    -- let the addon extend it with its own data
+    pfWorldMapFrame_Update()
+  end
 end)