Added a get_play_start_pts function to coincide with the
already-existing get_play_end_pts. This prevents code duplication
and also serves to make it so code that probes the start time
(such as get_current_pos_ratio) will work correctly with chapters.
Included is a bug fix for misc.c/rel_time_to_abs that makes it work
correctly with chapters when --rebase-start-time=no is set.