Support timelines that go backwards

[birtles]

The currently speced finishing behavior does not work well with timelines that go backwards.

From earlier email discussion with @theres-waldo:

Currently the things that get us out of the finished state are:

• setting the current time (which will cause us to reach the third case)
• setting the playback rate (which sets the current time)
• reversing (which sets the playback rate)
• setting the start time (which clears the hold time)
• updating the timing of the effect (which will cause us to reach the third case)
• cancelling (which clears the hold time)
• updating the timeline (which clears the hold time)

There may be others, but those are the ones that come to mind.

I guess we need to add something to account for timelines that can go backwards. Perhaps within the "update the finished state" procedure instead of using the current time, we need to manually calculate the current time ignoring the hold time.

Subsequent experimentation by @theres-waldo suggests that we can fix this if we ignore the hold time when calculating the current time in the "update the finished state" procedure but only if did seek is false.