fix issue #1309 - secret doors in Garden rooms

Issue reported by elunna:  when a room gets converted into a theme
room with fill type Garden, its walls are changed to trees but any
secret doors in those walls are still displayed as regular walls.

This adds a new D_ARBOREAL flag for secret doors, used to force them
to be displayed as a tree instead of a wall.

Fixes #1309

dat/themerms.lua

@@ -1071,7 +1071,10 @@ end
 -- postprocess callback: turn room walls into trees
 function make_garden_walls(data)
    local sel = data.sel:grow();
+   -- change walls to trees
    des.replace_terrain({ selection = sel, fromterrain="w", toterrain = "T" });
+   -- update secret doors; attempting to change to AIR will set arboreal flag
+   des.replace_terrain({ selection = sel, fromterrain="S", toterrain = "A" });
 end
 
 -- postprocess callback: make a trap