Move the flush request to the audio thread

src/common/serialization/clap.h

@@ -151,11 +151,7 @@ using ClapMainThreadCallbackRequest =
                  clap::ext::note_ports::host::SupportedDialects,
                  clap::ext::note_ports::host::Rescan,
                  clap::ext::params::host::Rescan,
-                 clap::ext::params::host::Clear,
-                 // This doesn't need to be done on the main thread, but we
-                 // don't have an alternative per-plugin instance socket
-                 // available so this is probably fine
-                 clap::ext::params::host::RequestFlush>;
+                 clap::ext::params::host::Clear>;
 
 template <typename S>
 void serialize(S& s, ClapMainThreadCallbackRequest& payload) {
@@ -177,7 +173,9 @@ void serialize(S& s, ClapMainThreadCallbackRequest& payload) {
  *       used.
  */
 using ClapAudioThreadCallbackRequest =
-    std::variant<WantsConfiguration, clap::ext::tail::host::Changed>;
+    std::variant<WantsConfiguration,
+                 clap::ext::params::host::RequestFlush,
+                 clap::ext::tail::host::Changed>;
 
 template <typename S>
 void serialize(S& s, ClapAudioThreadCallbackRequest& payload) {

src/common/serialization/clap/ext/params.h

@@ -223,8 +223,6 @@ struct FlushResponse {
     }
 };
 
-// TODO: This can be either a main thread or an audio thread function call
-//       depending on whether or not the plugin is active
 /**
  * Message struct for `clap_plugin_params::flush()`.
  */