codecs: Virtual functions should return GstFlowReturn, not boolean value

The return type of most virtual functions (except for output_picture) are boolean, not GstFlowReturn which looks bad design because baseclass doesn't understand why subclass returned false. Then baseclass will return GST_FLOW_ERROR whatever the reason was.

I'd suggest updating all virtual functions so that all of them can return GstFlowReturn, as all methods will be called in GstVideoDecoder::handle_frame()