Add AudioStreamPolyphonic to simplify sound playback from code

[reduz]

• This new audio stream allows to play multiple sounds and control them over time from code.
• It greatly simplifies tasks such as generative music (music generated from code) or audio.

This new type of stream was added with the goal of fixing audio blending in AnimationPlayer and AnimationTree, but can be used by others for their regular audio needs.

Does not fix anything currently, but should help implement #69758 properly.

Some demo code of how to use this:

var player = $SomeNode as AudioStreamPlayer
player.stream = AudioStreamPolyphonic.new()
var playback = player.get_stream_playback() as AudioStreamPlaybackPolyphonic
var id = playback.play_stream(preload("res://Clip1.ogg"))
await get_tree().create_timer(1).timeout
playback.set_stream_volume(id,-6) # Set volume to half after one second
await get_tree().create_timer(2).timeout
var id2 = playback.play_stream(preload("res://Clip2.ogg")) # 2 seconds later, start another clip
await get_tree().create_timer(1).timeout
playback.stop_stream(id) # 1 second later, kill the first clip
playback.set_stream_pitch_scale(id2,1.5) # Make the second clip go 50% faster

[Calinou]

Does the AnimationPlayer's audio playback tracks support this new stream? I've tried to use AnimationPlayer with AudioStreamPlayer as part of a demo project recently, but found that it didn't support polyphony (old sounds were replaced by new sounds as they were played, even if Max Polyphony was increased above 1 on the AudioStreamPlayer).

[reduz]

@Calinou So far this PR does not change animation code, but the idea is that it blends the audio from one animation to another (or lets it finish instead of killing it). Currently only one stream can be played at the same time always.

[TokageItLab]

Previously, the streams in AudioTrack were set directly to AudioStreamPlayer.

If I understand correctly, after this PR, we can just set AudioStreamPolyphonic which is made by AnimationPlayer/Tree internally to AudioStreamPlayer once. Then AnimationPlayer/Tree will handle the AudioStreamPolyphonic via AudioStreamPolyphonicPlayback to play multiple streams.

@reduz Is it right? If no problem, I will try to work on it later.

[reduz]

@TokageItLab that sounds great!

[TokageItLab]

We have been explained this implementation at ContributorsChat and I think that it seems there is no missing for solving #69758.

After this is merged, AnimationPlayer and AnimationTree will need to be changed to use this.

[akien-mga]

Thanks!