Subclasses should call this when they have prepared the
buffers they will aggregate for each of their sink pads, but
before using any of the properties of the pads that govern
*how* aggregation should be performed, for example z-index
for video aggregators.
If gst_aggregator_update_segment() is used by the subclass,
it MUST be called before gst_aggregator_selected_samples().
This function MUST only be called from the #GstAggregatorClass::aggregate()
function.
Subclasses should call this when they have prepared the buffers they will aggregate for each of their sink pads, but before using any of the properties of the pads that govern *how* aggregation should be performed, for example z-index for video aggregators.
If gst_aggregator_update_segment() is used by the subclass, it MUST be called before gst_aggregator_selected_samples().
This function MUST only be called from the #GstAggregatorClass::aggregate() function.