loopback: Improve latency estimation in snapshots

loopback: Improve latency estimation in snapshots

The delay and render memblockq are using the source and sink sample specs,
so using pa_bytes_to_usec() will produce better estimates of the delays than
using pa_resmpler_result(). Because the delays are considered to be part of
the sink or source latency, they are added to them. source_output_buffer
becomes obsolete.