SDL3: Switch audio system to use callbacks

[RobLoach]

This switches the SDL3 Audio System to use event callbacks rather than constant polling. There are a few benefits out of this over what was there before...

1. Wait: Before when the buffer was full, it waited in a loop, sleeping 1 millisecond each iteration. Now instead of a 1ms loop, it registers a wait condition, meaning it'll fill the buffer precisely when it's available. Also makes sure it's on the right thread.
2. Device Removal: Before if a device was lost, like a USB mic was removed, it would keep writing the audio device. Now there's a device removed event that will attempt to re-open the device, or eventually give up if it can't re-find it.

Test

1. Switch your audio device to sdl3, for both audio and mic
2. Load a core that plays some sounds, like audio_microphone
3. Expect to be able to record and hear sounds