Lua/ysdl version To-Do List:
----------------------------
  - cursor should also show red for insufficient money to buy tower
  - fix inefficient render.lua inner tower/enemy/projectile loop
  - enemy3 doesn't have diagonal images
  - weird: under linux, cursor transparency is bizarre cross-hatch (if alpha is 128)
  - 'local' could be used a lot more at file scope to speed up
      things which are only accessed within that one file
  - go through the code looking for causes of memory churn, in
      particular, the frequent creation/deletion of SDL_Rects could
      be avoided by the use of a few global rects, and the
      tower_definition().price thing is another example.

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From the original 0.3 TODO file:
--------------------------------
Roadmap for AWESOME TOWER DEFENSE:

0.4:
    *   Draw a circle which indicates the range of the tower when selected, 
        and when you want to buy one.
    *   Code cleanups.
    *   New enemy sprites.
    *   3 new towers.
    *   25 new batches of enemies in level 1 (currently, out of 1)

0.5:
    *   Implement local highscores.
            -   For linux, I'll implement the /var/games/ correctly.
    *   Add all sorts of statistics about the game.

0.6:
    *   Implement online highscore.
    *   New level, 50 new batches.

0.7:
    *   Level editor.
    *   New level, 50 new batches.

0.8:

0.9:

1.0:
    *   Feature complete at this point. Only bugfixes from now on.
