The stats are a superset of the quality factor, also allowing the picture type and encoder "PSNR" stats to be exported
This also replaces the native by fixed little endian order for the affected side data

AV_PKT_DATA_QUALITY_FACTOR is left as a synonym of AV_PKT_DATA_QUALITY_STATS
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>