Simplify advance method on SpanTracker

[Keinsleif]

Similar code appear on some sources.

let current_sample_rate = self.input.sample_rate();
let current_channels = self.input.channels();
let input_span_len = self.input.current_span_len();

I think this is not clean.

I used code similar to the seek method to fix this.
Now, advance method receive reference for Source.

Also, channel count and sample rate are not used on loops that are not boundaries.
In this PR, SpanTracker retrieves them from input source only on boundaries.

[Keinsleif]

I'm not very familiar with how SpanTracker works, but is this working correctly?