AudioProcessorARAExtension: Add support for ARAPlaybackRenderer::Alwa…

[pierreguillot]

The prepareToPlay() methods of the juce::ARAPlaybackRenderer and juce::ARAEditorRenderer classes take ARAPlaybackRenderer::AlwaysNonRealtime as an argument, but the juce::AudioProcessorARAExtension::prepareToPlayForARA() method ignores this argument and defaults to false. This PR allows you to control this argument and set it according to juce::AudioProcessor::isNonRealtime() for example. This can be used to decide which type of juce::AudioFormatReader (Buffering or not) to use, for instance.