Various cleanups and streamlining of interfaces as precursor
to further advancements in rate control.
Pre-encode parameter setting for different use cases:
One-pass, first of 2-pass, second of 2-pass, and Svc
are separated out.

There is no change in output with this change.

Change-Id: Ied8ca7d84d610993776aa30ef263fe20452e0e3e