Clean up of 3D->2D conversion.

Clean up of 3D->2D conversion.

It makes slightly more sense to discard 2D in the Transcoder
rather than the Encoder.  Unfortunately this requires quite
invasive changes, mainly to remove Encoder::_position and instead
derive this information from the PlayerVideo that is being handled.
This is also nicer than before, I think.

A notable change is that in player.cc; using time rather than
content_video_to_dcp().  This means we are assuming that the decoder
returns video at the time we ask it to, rather than checking
what it has returned.  I can't think of a problem with this (yet).