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]

pegs.c

@@ -936,8 +936,7 @@ static void game_set_size(drawing *dr, game_drawstate *ds,
 
     assert(TILESIZE > 0);
 
-    if (ds->drag_background)
-	blitter_free(dr, ds->drag_background);
+    assert(!ds->drag_background);      /* set_size is never called twice */
     ds->drag_background = blitter_new(dr, TILESIZE, TILESIZE);
 }