basevideodecoder: invoke subclass start method at state change and use set_format

basevideodecoder: invoke subclass start method at state change and use set_format

While this changes API slightly (e.g. actually uses set_format now), 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
* otherwise no subclass method at state change (global activation time)
Moreover, subclassese are either unaffected or trivially adjusted accordingly.