pipeline: flush after publishing call

Until now, the pipeline was never flushed out, so no flush() method from any
transformer was called. Let's fix that by creating a context publisher that
flushes once we finished publishing.

Change-Id: I2c0ab3c7c4aee77a1d7a1a6fccb19504c05f77f1
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "pipeline: flush after publishing call"