Add support for instrument plugins that require event loops to run on start.

[psobot]

Fixes #233.

The VST3 API allows plugins to assume that an event loop is always running; and as such, many plugins add callbacks to that event loop to handle asynchronous tasks like resource loading. Pedalboard doesn't pump this event loop (unless a GUI is being shown) and so these plugins may never get their callbacks, which usually results in them rendering silent audio when process(...) is called.

This PR adds a workaround: when loading an instrument plugin, we now perform a quick test at the end of the plugin's load function. We send a single MIDI note (channel 1, middle C, full velocity) and pump the event loop until we get audio back, or until we hit a 5-second timeout.

When tested with a variety of instrument plugins, plugin load time seems to increase by between 0ms and 250ms on my machine - but crucially, we no longer render silent audio for these plugins.

[drubinstein]

Makes sense. Can you make the warm up optional or make timeoutMilliseconds configurable from the Python API?

I'm curious if you could decrease warm up time by choosing a smaller bufferSize or sampleRate.