Returns the currently prepared video frame that has to be aggregated into
the current output frame.
This must only be called from the #GstVideoAggregatorClass::aggregate_frames virtual method,
or from the #GstVideoAggregatorPadClass::prepare_frame virtual method of the aggregator pads.
The return value is only valid until #GstVideoAggregatorClass::aggregate_frames or #GstVideoAggregatorPadClass::prepare_frame
returns.
Returns the currently prepared video frame that has to be aggregated into the current output frame.
This must only be called from the #GstVideoAggregatorClass::aggregate_frames virtual method, or from the #GstVideoAggregatorPadClass::prepare_frame virtual method of the aggregator pads.
The return value is only valid until #GstVideoAggregatorClass::aggregate_frames or #GstVideoAggregatorPadClass::prepare_frame returns.