demux_lavf: for avi, use packet .dts instead of .pts
When playing avi files take timestamps from the .dts field of the
packet instead of .pts. For avi libavformat returns the original avi
timestamps in .dts; without GENPTS set .pts is sometimes unset, with
GENPTS set it'll contain some made up values which are not correct
pts. Current libavformat also has a bug take makes it loop over the
whole avi file if you use GENPTS with video streams disabled.
The timing code can cope with the avi timestamps even though they're
not really pts.