[PiSink] Fixes for underrun, passthrough and multichannel

The current PiSink has a few issues that these commits address.
It fixes some high cpu and underrun issues.
It fixes passthrough of DTS/AC3.
It fixes the speaker mapping of multichannel.

It has been tested for a few weeks in Milhouse and Miappa test builds with positive results.
I think it is suitable for Gotham.