GUI app is spinning even when idle

[sowbug]

I think we need two kinds of channels, one async in the subscription, and one sync (but guaranteed not to block) in the background.

[sowbug]

What's going on:

• I added a canvas experiment and was naively clearing it on every window frame. This is a known issue.
• MidiSubscription is being dumb. It loops while waiting for an event, and it sleeps a little in between.
• Same with EngineSubscription, but the problem is more complicated because cpal should be telling it when to create another buffer of audio.

Solution:

• I have disabled the canvas experiment.
• I'm going to try fixing MidiSubscription first. My plan is to create a signal of some sort (probably std::sync::Condvar) and teach everyone to use it when they have something for the service.
• If that works, then I'll do it for the engine.