Prepares next pileup position in bam records collected by bam_plp_auto -> user func -> bam_plp_push. Returns
pointer to the piled records if next position is ready or NULL if there is not enough records in the
buffer yet (the current position is still the maximum position across all buffered reads).
Prepares next pileup position in bam records collected by bam_plp_auto -> user func -> bam_plp_push. Returns pointer to the piled records if next position is ready or NULL if there is not enough records in the buffer yet (the current position is still the maximum position across all buffered reads).