Explicit track cloning

Hi

I got the task from the editor's team to sum up the discussion around 
explicit track cloning from [1] and propose some changes.

Summary:
- we want explicit cloning.
- we already have a one to many relationship between sources and track 
instances
- we already have a one to many relationship between track instances and 
consumers (via MediaStream)

What wasn't entirely resolved:
- when addTrack() and the MediaStream() constructor doesn't auto-clone 
anymore, is it ok to add a track instance to more then one MediaStream?

API impact:
----------------
If we think it's ok to not put any limitations on adding the same track 
to several streams the API could behave as following (*1*):

Adopts the track instance (i.e. no cloning):
* MediaStream() constructor
* MediaStream.addTrack()

Clones the track instance:
* MediaStreamTrack.clone()

The current MediaStream() constructor is optimized for cloning an entire 
MediaStream since we identified that as a common use-case. Therefore, we 
could also have a MediaStream.clone() if we still want to have that 
operation as a one-liner.
----------------

If we think it's not ok for two track instances to live in different 
MediaStreams then we need to patch the above API a bit.

We've had two proposals to do this. One is to still have the addTrack() 
method clone the added track, but rename it to make this more clear 
(*2*). Unless that method would return the cloned track, we would still 
have the problem of finding a reference to the added track. It's however 
not that obvious how to patch the constructor in a similar way.

The second proposal was to use the 
addTrack()/MediaStreamTrack.clone()-API mentioned above but, e.g., throw 
an exception when a track is added to a second stream (*3*). An readonly 
attribute on a track telling which MediaStream that it's currently 
belong to could help in this case.
----------------

So what should we pursue?

My preference would be (*1*) or (*3*).

[1] 
http://lists.w3.org/Archives/Public/public-media-capture/2013Mar/0004.html