The image cache can lose track of a live image that it probably shouldn't lose track of

[dnfield]

The following sequence would lead to this:

• Put an image into the cache and hold a listener
• Create a keepAlive handle on the completer once the image completes.
• Remove the listener. At this point, the "last listener" callback will fire and remove the live image tracker.

In fact, the image should still be considered live because the completer/image are actually still available somewhere. Some potential solutions:

1. Drop the whole listener callback, and just have the liveImage check if the completer is disposed or not when it's wanted. If it's disposed, drop the live listener from the cache. Need to make sure this doesn't accidentally retain the image, but I'm pretty sure it doesn't.
2. Only call the last listener callback if the stream is getting disposed.

I started looking at doing this in #66688 but it was further complecting things and since this behavior existed already (e.g. add a listener, remove the last listener, add a listener again) I decided to leave it for a separate patch.

[kf6gpe]

Provisionally assigning a P4. @dnfield If you're currently working on this, let's bump the priority as you see fit.