opusenc: only request and process 1 frame at a time

opusenc: only request and process 1 frame at a time

... since it is specified in _finish_frame that input buffer may be invalidated
after calling it, and is as such not reliably available for further encoding.

Also, requesting or allowing several frames is only useful if subclass intends
to process these "in 1 run" (as in, 1 output buffer), not for having another
(inner) loop in subclass where the baseclass one will do just fine.