video_output: fix comment and move do_snapshot

The comment /* Get the subpicture to be displayed */ was written twice
in places where no subpictures were involved. In addition, do_snapshot
was computed where the rendering date was computed although it was not
used there.

Move the do_snapshot computation just before it's used, replace the
misplaced comments by comment actually describing what's follow, and
rewrite the removed comment where spu_Render actually returns the
subpicture.