Notifying request listeners after the target is notified

This might not be what you would like the library to behave like, but I thought I'd submit it as a pull request since I went ahead and implentented cause we needed it.

It seems to me that once you get notified that an image is ready on a target. you would expect the target to already have received that image so you can act on it.

Let me know if this is completely different to what you envisioned.