Fix #824: add OfflineAudioDestinationNode

[rtoy]

Add OfflineAudioDestinationNode so we can specify that the channel
count and channel mode cannot be changed after construction. Also,
explicitly state that the channel count is the number of channels
specified by the constructor.

[rtoy]

PTAL.

This basically defines the offline destination node.

I'm not sure, but think to do this right, we need to change BaseAudioContext so that the destination attribute is a BaseAudioDestinationNode and then define that AudioDestinationNode and OfflineAudioDestinationNode are subclasses. We can't have OfflineAudioDestinationNode be a subclass of AudioDestinationNode because the latter has a maxChannels attribute that doesn't make sense for an OfflineAudioDestinationNode.

[rtoy]

Well, maybe for simplicity (?) we could have maxChannelCount be equal to channelCount. Then OfflineAudioDestinationNode could be a subclass of AudioDestinationNode.

[joeberkovitz]

@rtoy I do not think we even need a new subclass here. We can instead specify that 1) maxChannelCount is always channelCount, and 2) the AudioDestinationNode for an OfflineAudioContext throws NotSupportedException on any attempt to modify its channelCount.

[rtoy]

I'm ok with either approach.

It was suggested by @hoch in #824 (comment) and agreed upon by @padenot to create an OfflineAudioDestinationNode. I have a slight preference to make OfflineAudioDestinationNode a subclass of AudioDestinationNode and making maxChannels be set to number of channels used to create the offline context.

[joeberkovitz]

Your simplified approach to an OfflineAudioDestinationNode subclass is likewise fine with me.

[joeberkovitz]

This looks good to me. I'm not sure if it's ready for merge...?

[rtoy]

No, it's not yet ready to merge because the current text doesn't define OfflineAudioDestinationNode as a subclass of AudioDestinationNode. Plus, need to clarify maxChannels for OfflineAudioDestinationNode.

[rtoy]

If #968 is implemented, then there isn't really a need for a separate OfflineAudioDestination.

[rtoy]

Closing this. The restriction on the channel count and mode will be handled when #968 is fixed.