Only set up VST3 SHM audio buffers in setActive()

This avoids doing the duplicate check (since both `setProcessing()` and
`setActive()` would be called), and this also gets rid of the assumption
added a couple commits ago that `setupProcessing()` is only ever called
once, which is not true.

src/plugin/bridges/vst3-impls/plugin-proxy.h

@@ -468,7 +468,7 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
      * amount of copies required to only once for the input audio, and one more
      * copy when copying the results back to the host.
      *
-     * This will be set up during `IAudioProcessor::setupProcessing()`.
+     * This will be set up during `IAudioProcessor::setActive()`.
      */
     std::optional<AudioShmBuffer> process_buffers_;