Previously, if `AudioProcessorGraph::prepareToPlay` was called twice,
interspersed with calls to `setProcessingPrecision`, the graph would
consider the nodes 'prepared' on the second call, and wouldn't
re-prepare the inner nodes with the new precision setting.

    graph.setProcessingPrecision (juce::AudioProcessor::singlePrecision);
    graph.prepareToPlay (44100, 512);

    graph.setProcessingPrecision (juce::AudioProcessor::doublePrecision);
    graph.prepareToPlay (44100, 512); // this wouldn't update the nodes

Now, we always explicitly unprepare all nodes at the beginning of
prepareToPlay, so that they'll always receive the newest settings.

We also build this into the AudioPluginHost, as a convenient way to
check the midi outputs of plugins and devices.