Add maxChannelCount for MediaStreamAudioDestinationNode

[mjwilson-google]

Describe the feature
Currently we have maxChannelCount for AudioDestinationNode.

However, MediaStreamAudioDestinationNode does not have this attribute, so the channel count is always 2.

Perhaps we should also have a maxChannelCount here?

Is there a prototype?
No

Describe the feature in more detail
This would also require changing the constructor for MediaStreamAudioDestinationNode to allow specifying the channel count.

[mjwilson-google]

WG Meeting 2025-07-10 Discussion:

• MediaStreamAudioDestinationNode isn't necessarily bound by the same constraints as AudioDestinationNode. There doesn't need to be a maximum unless there are hardware or CPU constraints.
• Since MediaStreamAudioDestinationNode doesn't have a channelCount constraint it may be implicitly possible to change the channel count. We should double-check the spec language related to this.

[mjwilson-google]

I checked Chromium's implementation, and we have a maximum of 8 channels based on our media stream source potentially ignoring more than 8 channels.

I'm not sure if this is a propagated constraint from MediaStream (in which case it's probably not necessary to specify directly in Web Audio but we may want to add a non-normative note), or if this is an implementation-specific behavior / bug and we really should allow an arbitrary number of channels.

@padenot would you be able to check Firefox's implementation and give an opinion?

[padenot]

@padenot would you be able to check Firefox's implementation and give an opinion?

Firefox's implementation of MediaStreamAudioDestinationNode isn't different from anything else in our Web Audio implementation, and our somewhat arbitrary limit of 32 channels is what limits it. Consumers of the MediaStream can provoke down-mixing however.