Right now those muxers use the default timebase in all cases(1/90000).

This patch avoid unnecessary rescaling and makes the printed timestamps
more readable.

Also, extend the printed information to include the timebases and packet
pts/duration and align the columns.

Obviously changes the results of all fate tests which use those two
muxers.

It's not supposed to be set outside of lavc. Set r_frame_rate and
avg_frame_rate instead.

Signed-off-by: Mans Rullgard <mans@mansr.com>