VST3: Allow plugins to designate the first input bus as Aux rather than Main

To use this feature, derive your AudioProcessor from
VST3ClientExtensions and override getPluginHasMainInput() to return
false. The main input bus will then be designated as an Aux bus, rather
than a Main bus.

This is mainly useful for synth plugins like vocoders, which may need a
sidechain audio input, but which should replace all audio on the channel
with the output of the synth, rather than mixing with the audio input.

modules/juce_audio_processors/utilities/juce_VST3ClientExtensions.h

@@ -82,6 +82,14 @@ struct VST3ClientExtensions
         called - this function may not be called at all!
     */
     virtual void setIHostApplication  (Steinberg::FUnknown*) {}
+
+    /** This function will be called to check whether the first input bus
+        should be designated as "kMain" or "kAux". Return true if the
+        first bus should be kMain, or false if the bus should be kAux.
+
+        All other input buses will always be designated kAux.
+    */
+    virtual bool getPluginHasMainInput() const  { return true; }
 };
 
 } // namespace juce