Ability to play animations backwards in time

[kikito]

Right now the condition of dt being posititive is pretty much "hard-coded" into anim8. The culprint is the update method:

function Animation:update(dt)
  if self.status ~= "playing" then return end

  self.timer = self.timer + dt

  while self.timer > self.delays[self.position] do
    self.timer = self.timer - self.delays[self.position]
    self.position = self.position + self.direction
    if self.position < 1 or self.position > #self.frames then
      self:endSequence()
    end
  end
end

It would be cool if animation:update admitted negative dts. This way we could use it in games that involve time manipulation (like in Braid).

[kikito]

anim8 is able to run animations backwards since v2.0