Use child duration to calculate switch position

[shamansir]

@skavish This one could work better, but it requires clip children to be exported with appropriate bands.

Editor currently works like this:

Clip:     ♦--Rect--◇------♦---Circle---♦--Rect--◇--------◇----♦
  Rect:   ♦-trans.-♦--no-tweens----------------∞ (Rect band is exported as [0, Infinity])
  Circle: ♦--rotate----♦--no-tweens----------------∞ (Circle child band exported as [0, Infinity])

◇ is the place where Rect time is reset to 0 in Editor. I need this band to be able to do the same in Player.