Strip AudioStream WAV/Ogg/MP3 data using placeholders for dedicated server exports

[Calinou]

• Follow-up to Strip VoxelGIData and LightmapGIData using placeholders for dedicated server exports #112744 (can be merged independently).

This greatly reduces PCK size for dedicated servers in projects that use audio in WAV, Ogg Vorbis and MP3 formats. Additionally, this prevents errors from being printed on export when using --headless to export a dedicated server PCK in a project that uses a VoxelGI node.

• This closes Strip audio from dedicated server exports to reduce PCK size godot-proposals#12748.

Testing project: test_dedicated_server_export_2.zip

Preview

File sizes are in bytes. The test project includes 3 short sound files (2.25s each): one WAV, one Ogg Vorbis and one MP3 sound.

Before this PR, both PCK/ZIPs would be the same size as the project doesn't include any textures.

PCK size

194216 /tmp/out.pck
  7228 /tmp/out_server.pck

ZIP size

172691 /tmp/out.zip
  6283 /tmp/out_server.zip

TODO

• Fix audio playback length not being respected. In the current implementation, the playback position never advances, which means AudioStreamPlayer never emits the finished signal. We want this signal to be emitted at the right time just like in a standard build, as it may be relied upon server-side for gameplay purposes.
  • Before I had mix() implemented (returning -1 instead), it made AudioStreamPlayer emit its finished signal instantly instead.
  • I'm not sure how to implement this in the simplest way, while still respecting loop mode/loop begin/loop end (as these properties affect when the finished signal is emitted). The AudioStreamPlayer's playback_speed property should also be respected, as well as manual seeking.
• Change loop_begin and loop_end to be stored in seconds as opposed to samples, as there's no concept of samples in PlaceholderAudioStream (we don't store the sound's original sample rate).