Feature request: get_volume() for StaticSoundHandle and StreamingSoundHandle

[GitGhillie]

get_volume() would get the volume as set by set_volume().

Other audio libraries seem to have this in their API:
https://www.fmod.com/docs/2.00/api/studio-api-eventinstance.html#studio_eventinstance_getvolume
https://docs.rs/rodio/latest/rodio/struct.Sink.html#method.volume

And it could be helpful in situations where the volume is set from different places.

[ItsDoot]

Alternatively, if we could pass a closure to set_volume() of the form |cur_vol: Volume| calculate_new_volume(cur_vol), that would cover the most notable usecase(s).

[tesselode]

I've discussed this a bit in the Bevy discord, but there's some considerations for a feature like this:

• Which volume do we report - the actual volume as last reported from the audio thread, or the one last set by the user? (Or both?)
  • If it's the former, I'd want to make sure this doesn't have significant performance impacts
  • If it's the latter, it's worth keeping in mind that the last-set volume won't always be a number. It could be Value::FromModulator, in which case, you wouldn't just be able to increment or decrement that value.
• Do we even need this function? You can track the last-set value yourself, although you can't track the current volume unless the audio thread reports it.

I don't want to add this unless the pros outweigh the cons (performance, potential disappointment/confusion when people realize they can't rely on get_volume returning a number). The pros seem minimal to me currently, and I have no idea what the cons are.