Correct the initialization of the variable "firstTick"

[Pointy]

In the "tick" handler code, there's a flag called firstTick that is used to avoid updating a property value that's already correct for the current tween. However, firstTick is initialized based on the state of the timeStart value, which is unset on the first tick; the initialization incorrectly inverts that test, using !!timeStart instead of !timeStart.

The upshot of this is that the test to avoid updating a property an all but the first tick actually only avoids updating the property on the first tick; for most of the animation that optimization is ignored. Worse, that can cause issues for force-fed property values that might need initialization on that first tick (see issue 543).

[Rycochet]

Ugh, good catch - will get this merged when v2 is out and v1 moves into a maintenance branch, thanks! :-)