Added transparent rendering of child Renderables, added timewarp support
for animations.

- Renderables now have two extra attributes: children and cascade.
  children holds all the child renderables of the renderable, cascade
  indicates whether or not the engine should handle the rendering,
  transforms and animations of those children automatically.
- The Animation constructor now has an extra argument: time_warper,
  which is expected to be a function taking a value 0..1.0 and returning
  a warped time value 0..1.0.
- Fixed some animation bugs.