Calling setOnVideo/AudioFrameCallback outside of the onTrackSubscribed handler never starts a frame thread handler

[Soralsei]

LiveKit C++ SDK Version

v1.2.0

Platform

Linux x86_64

OS Version

OpenSUSE 15.6 LEAP

Compiler & Version

gcc 15.2

CMake Version

3.28.3

How are you using the SDK?

Vendoring the SDK and building from source as a CMake subdirectory

Issue Description

Calling setOnAudio/VideoFrameCallback correctly adds the frame handler to the relevant video/audio_callbacks, but doing so outside of the onTrackSubscribed delegate callback never starts a reader thread, resulting in the corresponding frame callback never being called.

This happens because SubscriberThreadDispatcher::handleTrackSubscribed only gets called synchronously right after the delegate callback from the Room event handler @ room.cpp:703, which no-ops if the frame callback key has not been added to the callback map yet.

A potential fix could be to call SubscriberThreadDispatcher::handleTrackSubscribed if setOnAudio/VideoFrameCallback is called when the mappings are empty.

Steps to Reproduce

1. Handle the onTrackSubscribed room delegate event
2. Call Room::setOnVideoFrameCallback from the context of another thread (ex: in a Qt GUI thread) that can run after the event handler returns
3. Notice no audio/video frame callbacks being called