process effect-automation when bouncing w/processing
allow to set custom thread-buffer size

This is needed for gain and pan automation buffers
as well as silent and scratch buffers when bouncing or
exporting with larger chunk size than the current engine
period.
apply gain automation on bounce - fixes 5887
fixup freeze operation.

since rev. 1e3a955, Session::write_one_track() gets and drops
thread buffers.