[Web] Restore ScriptProcessorNode audio driver fallback

Godot has a ScriptProcessorNode audio driver implementation for the
(deprecated) Web API.

As reported by some users, this fallback was not properly re-added
during the Godot 4 transition, and was left as "dead code".

While the API is deprecated, it is still supported by most browsers, and
some WebView may not implement AudioWorklet correctly (the new
recommended API).

This commit re-adds the ScriptProcessorNode implementation as a fallback
if the AudioWorklet driver fails to initialized (and can be forced if
desired via project settings as usual).

platform/web/audio_driver_web.cpp

@@ -479,6 +479,8 @@ void AudioDriverWorklet::_capture_callback(int p_pos, int p_samples) {
 	driver->_audio_driver_capture(p_pos, p_samples);
 }
 
+#endif // THREADS_ENABLED
+
 /// ScriptProcessorNode implementation
 AudioDriverScriptProcessor *AudioDriverScriptProcessor::singleton = nullptr;
 
@@ -497,5 +499,3 @@ Error AudioDriverScriptProcessor::create(int &p_buffer_samples, int p_channels)
 void AudioDriverScriptProcessor::start(float *p_out_buf, int p_out_buf_size, float *p_in_buf, int p_in_buf_size) {
 	godot_audio_script_start(p_in_buf, p_in_buf_size, p_out_buf, p_out_buf_size, &_process_callback);
 }
-
-#endif // THREADS_ENABLED

platform/web/audio_driver_web.h

@@ -169,6 +169,8 @@ public:
 	AudioDriverWorklet() { singleton = this; }
 };
 
+#endif // THREADS_ENABLED
+
 class AudioDriverScriptProcessor : public AudioDriverWeb {
 private:
 	static void _process_callback();
@@ -178,7 +180,6 @@ private:
 protected:
 	virtual Error create(int &p_buffer_size, int p_output_channels) override;
 	virtual void start(float *p_out_buf, int p_out_buf_size, float *p_in_buf, int p_in_buf_size) override;
-	virtual void finish_driver() override;
 
 public:
 	virtual const char *get_name() const override { return "ScriptProcessor"; }
@@ -191,6 +192,4 @@ public:
 	AudioDriverScriptProcessor() { singleton = this; }
 };
 
-#endif // THREADS_ENABLED
-
 #endif // AUDIO_DRIVER_WEB_H

platform/web/js/libs/library_godot_audio.js

@@ -2160,7 +2160,7 @@ autoAddDeps(GodotAudioWorklet, '$GodotAudioWorklet');
 mergeInto(LibraryManager.library, GodotAudioWorklet);
 
 /*
- * The ScriptProcessorNode API, used when threads are disabled.
+ * The ScriptProcessorNode API, used as a fallback if AudioWorklet is not available.
  */
 const GodotAudioScript = {
 	$GodotAudioScript__deps: ['$GodotAudio'],

platform/web/os_web.cpp

@@ -275,6 +275,7 @@ OS_Web::OS_Web() {
 
 	if (AudioDriverWeb::is_available()) {
 		audio_drivers.push_back(memnew(AudioDriverWorklet));
+		audio_drivers.push_back(memnew(AudioDriverScriptProcessor));
 	}
 	for (AudioDriverWeb *audio_driver : audio_drivers) {
 		AudioDriverManager::add_driver(audio_driver);