Persistent sounds & threaded OpenAL

[Couls]

Summary

This PR adds an engine-level persistent sound capability and moves OpenAL buffer updates onto a dedicated background thread, so 2D streams (music, PDA radio) keep playing seamlessly across level transitions and during the load screen, and remain fully controllable from Lua afterward. mtSound remains a console command and can be set to 0 to disable multithreaded sound implementation

Motivation

In the engine, every emitter is destroyed when a level unloads (_clear), and the main thread is blocked for seconds during loading, so any streamed audio stops dead. Mods that want continuous music (e.g. a PDA radio) have no clean way to keep a stream alive and re-bind to it after the new level spawns. This PR solves both problems at the source.

What's included

1. Persistent emitters (survive level unload)

• CSound_emitter::set_persistent(bool) / is_persistent() — a persistent emitter is forced to 2D and is not destroyed during _clear.
• The core keeps a strong reference to the owner data (s_persistent_refs) so the stream survives Lua GC.
• During _clear, persistent survivors are detached from their render target (enter "simulating" state), kept in s_emitters, and their backing CSound_source is preserved while every non-persistent emitter/source is released as before.

2. Per-emitter "persist in menu" flag

• set_persistent_in_menu(bool) / is_persistent_in_menu() controls whether a given persistent emitter keeps playing while the game is paused in a menu, replacing the old global toggle. pause_emitters() honors it per-emitter.

3. Owner re-binding after transition

• After a level load recreates the Lua sound_object (new owner pointer) while the persistent emitter is still playing, the engine reconnects them: reconcile_emitter_feedback() re-links owner_data->feedback, matching by source handle when the owner pointer changed.
• New helpers: stop_emitters_for_owner(), has_playing_emitter_for_owner(), consolidated through a single find_emitter_for_owner() lookup.
• ref_sound::stop() / has_playing_emitter() now work even when the cached feedback pointer was lost across a load.

4. Threaded OpenAL update

• OpenAL buffer refills run on a dedicated SoundRender_UpdateThread; the main thread only publishes a listener snapshot, so audio does not stutter while the main thread is blocked loading a level.
• All AL access is serialized by a single critical section (sound_api_enter/leave, which also makes the AL context current per thread).

5. Console command

• snd_stop_persistent — force-stops all persistent emitters (cleanup / safety).

Lua API

The feature is exposed on the existing sound_object:

  local snd = sound_object("music\\my_track")
  -- Mark the stream persistent so it survives level transitions.
  -- arg1: sets to a persistent emitter across level load
  -- arg2 (optional): keep playing while paused in a menu (default true)
  snd:set_persistent(true, true)
  -- Play as a 2D stream on the music bus (s2d + sm_Intro flag = 2)
  snd:play(nil, 0, sound_object.s2d + 2)
  snd.volume = 0.5
  -- Query state (works even after a level change re-binds the emitter)
  if snd:playing() then ... end
  local p  = snd:is_persistent()
  local pm = snd:is_persistent_in_menu()
  -- Stop and release persistence
  snd:stop()
  snd:set_persistent(false)

[themrdemonized]

Add new lua exports into lua_help_ex please

[themrdemonized]

I got this crash while testing this PR. It might be related to relation between mtSound and mtSoundPlayer flag that is used for update_sound_player.

[themrdemonized]

another one. Happened when i shot a monster.

I'm marking as draft, too unstable for me

[Priler]

Hope this gon fix issue with rain sounds during loading screens when you alt+tab.
and some issues with the radio.