VST3 Client: Enforce that setActive is called before process

[Speechrezz]

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Opening some JUCE VST3s in FL Studio's patcher can cause crashes because process() may be called before setActive() (likely due to race condition in FL Studio).

This workaround adds an std::atomic<bool> flag which prevents process() from proceeding unless setActive() sets the flag to true.

Link to original issue: https://github.com/juce-framework/JUCE/issues/1193