Public issue https://github.com/mackron/miniaudio/issues/320

Public issue https://github.com/mackron/miniaudio/issues/319

This was happening specifically when uninitializing a data buffer while
it's still loading asynchronously.

This properly decouples data buffer nodes from data buffers and should
address some subtle bugs, especially with uninitializing data buffers
while they're still loading.

This also cleans up the decoding code, in particular by avoiding some
duplicate code between the synchronous and asynchronous decoding paths.

The loop points are specified relative to the range, and will be
clamped as such.

This change affects cases where a sound is loaded while the same sound
is still in the process of being decoded. It makes it so the ASYNC flag
will only wait for the underlying data buffer to be initialized,
whereas the lack of ASYNC flag will result in it waiting for the entire
sound to be fully decoded.