av_common: remove unneeded field

This is actually needed for encoding only, and not decoding. Drop it.
player: make the time display relative to start PTS

This commit makes the playback start time always at time 0.

Signed-off-by: wm4 <wm4@nowhere>
options: support setting start time relative to start PTS

Signed-off-by: wm4 <wm4@nowhere>
player: fix start position when specifying with percent