Re: [web-midi-api] Virtual MIDI ports (#45)

I agree about the latency, we need to take that into account. Some use cases:

 * Virtual instrument on a web page. Has inherent latency from the audio graph that the host needs to take into account. Needs a way to set the latency of its virtual input.
 * Web page that does sequencing. Needs to take the latency of an external instrument into account. Needs a way to read the latency of its output.
 * A web page that takes a guitar input and converts it to MIDI. Has latency in the D/A and pitch detection and needs to convey that to the consumer. Needs a way to set the latency of its virtual output.
 * A notation software. Needs to be able to sync various MIDI sources in order to get a synced composition. Needs a way to read the latency of its inputs.

So basically, I think what we need is for normal ports a way to read their latency (if not available, report 0) and for virtual ports to write their latency, e.g.

```java
partial interface MIDIInput {
  readonly double latency;
};

partial interface MIDIOutput {
  readonly double latency;
};

partial interface VirtualMIDIInput {
  double latency;
};

partial interface VirtualMIDIOutput {
  double latency;
};
```

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https://github.com/WebAudio/web-midi-api/issues/45#issuecomment-23284997