Implement blending audio feature to AnimationTree

[TokageItLab]

Fixed #65750.

Follow up #71855. Allows AnimationTree to blend audio using AudioStreamPolyphonic.

This PR adds the following options to the audio track:

• UseBlend
  • Sets whether the track will be blended with other animations. If true, the audio playback volume changes depending on the blend value.
• AutoClear
  • Sets whether the stream cache should be deleted when the blend amount is 0 in blending. If you want to fade in the audio or continue playback after blending, this value must be false. Note: Even if this value is false, the stream will be removed from the cache when the audio is played to the end. Removed, we will send an optimized PR to eliminate the need for this AutoClear in the future.

Known issue / Todo:

• The AnimationPlayer has also been changed to use AudioStreamPolyphonic, but audio blending is now only available in the AnimationTree due to AnimationPlayer blending and AnimationTree blending are inconsistent #70207. It will be available after the Implement AnimationPlayerBase class to make the blending process shared between AnimationPlayer and AnimationTree godot-proposals#5972 proposal is applied to the AnimationPlayer.
• Blending of the same animation with each other may result in a messed up audio. This is due to the fact that stream maps are assigned by the ObjectID of the Animation Resource, which may be resolved in the future if it becomes possible to assign arbitrary IDs to AnimationNodes.

Preview (Turn on the volume):

Godot.2023.01.28.-.15.17.34.15.mp4

audio_blend_test.zip

[TokageItLab]

After discussions with the @reduz, it was decided to remove the AutoClear option because it is hard to use and will likely not be used.

The AutoClear option was implemented to prevent audio with 0 amount blend from playing in the background, saving memory and saving audio polyphony counts. (For example, if a large number of animations are synced, there may be a large amount of audio playing in the background with a volume of zero.)

However, we have concluded that improvements to the AudioStreamPolyphonic API should prevent increasing the audio polyphony count in the background when the blend amount is 0 by properly pausing and playing back the audio.

This optimization will be implemented in the future as Follow up PR. Until then, we can have the workaround that increase the number of audio_max_polyphony property in AnimationTree to handle that.

[SaracenOne]

Yeah, this seems good to me!

[akien-mga]

Thanks!