Clear map.visible_tiles only once

It feels wrong to clear map.visible_tiles every time in the outer loop.  Also, what if there's a monster that's keeping track of its own viewshed, and it gets processed after the player?  The code would clear all map visible tiles and forget what the player can see.

book/src/chapter_5.md

@@ -486,7 +486,6 @@ Our creation method also needs to know to add all false to it, just like before:
 
 ```rust
 if viewshed.dirty {
-    for t in map.visible_tiles.iter_mut() { *t = false };
     viewshed.dirty = false;
     viewshed.visible_tiles.clear();
     viewshed.visible_tiles = field_of_view(Point::new(pos.x, pos.y), viewshed.range, &*map);
@@ -494,6 +493,7 @@ if viewshed.dirty {
     // If this is the player, reveal what they can see
     let _p : Option<&Player> = player.get(ent);
     if let Some(_p) = _p {
+        for t in map.visible_tiles.iter_mut() { *t = false };
         for vis in viewshed.visible_tiles.iter() {
             let idx = map.xy_idx(vis.x, vis.y);
             map.revealed_tiles[idx] = true;
@@ -551,4 +551,4 @@ If you `cargo run` your project, you will now have visible tiles as slightly cya
 
 Copyright (C) 2019, Herbert Wolverson.
 
----
+---