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
+     * Ask the VST plugin to process audio for us. If the plugin somehow does
-     * the deprecated 'process' function.
+     * 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,

src/wine-host/plugin-bridge.cpp

@@ -201,8 +201,6 @@ void PluginBridge::handle_dispatch() {
         request =
             read_object(host_vst_process_replacing, request, process_buffer);
 
-        // TODO: Check if the plugin doesn't support `processReplacing` and
-        //       call the legacy `process` function instead
         // The process functions expect a `float**` for their inputs and
         // their outputs
         std::vector<float*> inputs;
@@ -220,8 +218,22 @@ void PluginBridge::handle_dispatch() {
             outputs.push_back(buffer.data());
         }
 
+        // Any plugin made in the last fifteen years or so should support
+        // `processReplacing`. In the off chance it does not we can just emulate
+        // this behavior ourselves.
+        if (plugin->processReplacing != nullptr) {
             plugin->processReplacing(plugin, inputs.data(), outputs.data(),
                                      request.sample_frames);
+        } else {
+            // If we zero out this buffer then the behavior is the same as
+            // `processReplacing``
+            for (std::vector<float>& buffer : output_buffers) {
+                std::fill(buffer.begin(), buffer.end(), 0.0);
+            }
+
+            plugin->process(plugin, inputs.data(), outputs.data(),
+                            request.sample_frames);
+        }
 
         AudioBuffers response{output_buffers, request.sample_frames};
         write_object(host_vst_process_replacing, response, process_buffer);