new opacity animation issue for 0 millisecond duration

I often use animation durations of 0 milliseconds for my animateLayer() calls to avoid any unnecessary delays in my automated unit tests.  I've discovered an issue in your recent animation changes (starting with 5949ad2b90045d25186e61130d6f4f9d5ed7a4d3) that's occurring since the update across a lot of platforms (see "Platforms" below).

In my latest code update, see the following test:
   tests/by-hand/animateLayer/drawLinesInstantShiftVariedOpacity.html
Note that the 4th vertical green bar shifts left to right in position as expected, but the opacity fails to animate (it appears to remain at the initial 0.4 opacity).

Platforms:
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Android
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  Dolphin Browser 9.1.0
  Firefox 17.0

Linux
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  Arch 3.6.11-1
    Chrome 23.0.1271.97
    Chromium 23.0.1271.97
    Firefox 17.0.1
  Linux Mint 14
    Chrome 23.01.1271.97
    Chromium 22.0.1229.94
    Firefox 17.0.1
  Mageia 2
    Chromium 20.0.1132.57
    Firefox 10.0.11

Windows 7
---------
  Chrome 23.0.1271.97 m
  Firefox 17.0.1
  SRWare Iron 23.0.1300.0