Make the ABR buffer consider the distance to the end of the video

Make the ABR buffer consider the distance to the end of the video
Should improve bitrate accuracy in 2-pass mode.
May also slightly improve quality by allowing more variation earlier-on in a file.

Also fix abr_buffer with 1-pass: it does something very different than what it does for 2-pass.
Thus, the earlier change that increased it based on threads caused 1-pass ABR to be somewhat less accurate.