Take resampler delay into account when computing the buffer fill level,
including the fractional part.
If decode-buffer is now fed nominal packet reference times in
write_packet(), it converges the total buffer + resampler latency to the
target at sub-sample accuracy.
This is needed for aligning RX of ISO streams in the same group, so that
e.g. stereo pair alignment is achieved even though the streams have
separate resamplers. Resampler phases get aligned via independent rate
matching.