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/wine-host/bridges/vst3.h

@@ -171,7 +171,7 @@ struct Vst3PluginInstance {
      * A shared memory object we'll write the input audio buffers to on the
      * native plugin side. We'll then let the plugin write its outputs here on
      * the Wine side. The buffer will be configured during
-     * `IAudioProcessor::setupProcessing()`. At that point we'll build the
+     * `IAudioProcessor::setActive()`. At that point we'll build the
      * configuration for the object here, on the Wine side, and then we'll
      * initialize the buffers using that configuration. This same configuration
      * is then used on the native plugin side to connect to this same shared