fix: report exhaustion for Stoppable and Skippable

[roderickvd]

When a source was stopped or skipped, Stoppable/Skippable continued to report a non-zero current_span_len(), so is_exhausted(). This caused queue to incorrectly bootstrap the mixers UniformSourceIterator with the old instead of the new sample rate.

Fixes what @sandreas reported in #853, although it's an issue distinct from get_pos().

[yara-blue]

new tests nice!

[sandreas]

@roderickvd

Thank you very much, I can now report, that the problems seem to be fixed. I tried a sample with my Test SineWave followed by the low samplerate m4b file and everything works as expected. Here and there I get a "buffer overrun/underrun" message, but the playback continues.

Thank you very much.