Expose a way to implement a backoff protocol, refactor errors and other minor improvements

[ferjm]

This PR exposes the EnoughData and NeedData Player events that allow consumers to implement a backoff protocol.

I made a couple of significant refactors:

• I moved the list of Player observers and renderers out of PlayerInner to avoid deadlocks. For more information, you can read this IRC conversation.
• I had to change the way we report errors in general to allow external consumption.

[ferjm]

r? @Manishearth

[ceyusa]

what worries me about this change is that we are exposing a gstreamer specific behavior, when the interface is supposed to be generic.

[ferjm]

Well, despite the event name choice, I don't think this is a Gstreamer specific behavior. What we are exposing is a way for the Player to control the flow of data that it receives. I expect other backends to have equivalent needs, and if they don't, we can still just signal the need of data.

Anyway, I am open to suggestions about how to implement a backoff protocol in a different way :)

[Manishearth]

do both the lists and the individual renderers need to be behind Arcs and mutexes? I feel like having Vec inside the list may be fine here?

[ferjm]

Each individual renderer is accessed from the layout and gstreamer threads, so I think we need to have them behind Arcs and mutexes.

[Manishearth]

ah, okay then. r=me