Hold lock for the whole of Butler::video(). Fixes jumpy nudge-seek

Hold lock for the whole of Butler::video().  Fixes jumpy nudge-seek
when playing DCPs.

Before, the lock was only held to check _pending_seek_position.
With that arrangement, this sequence was possible:
1. video arrives, check _pending_seek_position, decide it's ok, release lock
2. Butler::seek called, sets _pending_seek_position, clears _video
3. video that arrived in #1 is put into _video by ::video()
4. that video is given to an awaiting get_video() call.
This clearly subverts the attempts not to accept new video data while
a seek is pending.