'bottom' variable removed from sprite

Sprites are rendered in order of increasing y-coordinate, so that sprites
below other sprites are rendered on top. This has caused problems with the
tile system in the past, because tiles used to change their y-coordinates
arbitrarily so collision would work. Because of this, a 'bottom' variable
was added so that tiles were always rendered first. Now that the collision
system has been replaced, and is no longer dependent on this arbitrary
change of position, the y-coordinate stays at 0, and so the 'bottom'
variable is no longer needed. This variable has now been removed.
Audio synced with input

The audio server has been on the top of the render stack, to keep it from
being blocked. This is problematic, though, because it means that all audio
control from each frame doesn't execute until the beginning of the following
frame, which causes a noticable sync issue. Now, the audio server runs after
the render stack. This way, it never gets blocked, but it also executes in
the proper frame.