Add gst_vaapi_encoder_set_bitrate() interface to allow the user control
the bitrate for encoding. Currently, changing this parameter is only
valid before the first frame is encoded. Should the value be modified
afterwards, then GST_VAAPI_ENCODER_STATUS_ERROR_OPERATION_FAILED is
returned.
https://bugzilla.gnome.org/show_bug.cgi?id=719529
Fix the GstVaapiEncoderClass parent class type. Make sure to validate
subclass hooks as early as possible, i.e. in gst_vaapi_encoder_init(),
thus avoiding useless run-time checks. Also simplify the subclass
initialization process to be less error prone.
