Fix exceptions where HTMLMediaElement loads and plays race

[ara4n]

No description provided.

[kegsay]

Why are you attaching this to the prototype rather than being an instance member of the MatrixCall like the other fields are?

[kegsay]

Also, we're using this as a dictionary where an external entity (the caller) can set arbitrary keys. We should make this a proper map then using Object.create(null) (an object without a prototype) else you can get fun bugs if people decide to use "hasOwnProperty" as a queue ID. It's unlikely to happen in this particular case, but given it's a direct replacement and everything else remains the same, it's trivial to do.

[kegsay]

LGTM aside from comments.

[kegsay]

LGTM

[ara4n]

ironically there was a horrific bug here; by discarding failed promises, the whole promise chain will break forever if a play() fails. Turns out this is quite common, as things like unmounting the <video/> tag whilst its play() is waiting for media to arrive causes the play()'s promise to fail. This is easy to do when toggling between rooms whilst waiting for calls to start, etc.

Hopefully this is fixed now and so going to merge (with a bunch of debugging in case of future problems)