Add support for legacy plugins with only process()

By emulating the behavior of `processReplacing()`. Not that there should
still be any plugins around that don't support processReplacing.

src/plugin/host-bridge.cpp

@@ -357,7 +357,7 @@ void HostBridge::process_replacing(AEffect* /*plugin*/,
     const AudioBuffers request{input_buffers, sample_frames};
     write_object(host_vst_process_replacing, request, process_buffer);
 
-    // /Write the results back to the `outputs` arrays
+    // Write the results back to the `outputs` arrays
     AudioBuffers response;
     response =
         read_object(host_vst_process_replacing, response, process_buffer);

src/plugin/host-bridge.h

@@ -78,8 +78,10 @@ class HostBridge {
                       void* data,
                       float option);
     /**
-     * Ask the VST plugin to process audio for us. This should also be used for
-     * the deprecated 'process' function.
+     * Ask the VST plugin to process audio for us. If the plugin somehow does
+     * not support `processReplacing()` and only supports the old `process()`
+     * function, then this will be handled implicitely in
+     * `PluginBridge::handle_process_replacing()`.
      */
     void process_replacing(AEffect* plugin,
                            float** inputs,