I've dithered a bit in the past about whether or not it's allowable

to call game_set_size() twice on the same drawstate. Finally, a
definite decision: it isn't. Accordingly, midend.c arranges never to
do so, the devel docs state that puzzles may enforce by assertion
that it never happens, and the four puzzles which care (i.e. use
blitters) do so.

[originally from svn r6274]

map.c

@@ -2505,8 +2505,7 @@ static void game_set_size(drawing *dr, game_drawstate *ds,
 {
     ds->tilesize = tilesize;
 
-    if (ds->bl)
-        blitter_free(dr, ds->bl);
+    assert(!ds->bl);                   /* set_size is never called twice */
     ds->bl = blitter_new(dr, TILESIZE+3, TILESIZE+3);
 }