[Triks-C] Allow sending of data when output thread is not spun up.

This fixes an issue with final blanking data not being sent at the end
of a playlist because the output thread has already been spun down once
the last sequence ended.
[FPD] Create a separate output thread now that SPI writes are blocking

SPI writes used to complete almost immediately, but now do not return
until the data is completely written.  Since this could delay sending
data to other channel outputs, we thread off the SPI writes for the FPD
channel output to allow other channel outputs to run in parallel.