add point-to-point encoding

add point-to-point encoding

allows frame and pts based start points.  end points were already
previously supported.

New job variables pts_to_start and frame_to_start specify the start point.
There can be a period during the encode where it has to search for
the start point. During this period, libhb sets a new state
HB_STATE_SEARCHING and sets progress and eta till start point found.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@3039 b64f7644-9d1e-0410-96f1-a4d463321fa5