basevideoencoder: invoke subclass start method at state change

basevideoencoder: invoke subclass start method at state change

While this changes order w.r.t. set_format, which is OK for unstable API,
it has following merits:
* symmetric w.r.t. stop at state change
* in line with other base class practice
* little benefit in invoking 2 subclass virtual methods (set_format and start)
  in immediate succession; all actions in the second could be done in the first
  whereas subclass has no chance to do anything 'global' at activation time
Moreover, current -bad subclass relevant methods either trivially commute
or are either trivially adjusted accordingly.