more optimizations

LessonButton has a static var for its sprite sheet now, can't believe I
missed that.
Got rid of some push() calls in MainMenu init().
Got rid of any calls to Vector.length in CardStack, it's now all handled
manually for performance.
Got rid of pop() in Deck.getTopCard(), cards in the CardStack are now no
longer delete, but instead we don't consider them in the valid range
(aka garbage memory to be overwritten later).