Adds two new parameters to hb_scan, to control the number of preview frames generated during scan, and whether or not they're written to disk for later display. This will break any interfaces that use hb_scan until the new params are specified...sorry. Also adds a new job->seek_points setting (set this to the same as the number of previews) to be used with job->start_at_preview when doing live preview encodes, so the seek function has a frame of reference.
Wires up the CLI with a --previews option (long option only) to control the new scan parameters, and defaults the CLI to not writing previews to disk.
git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1970
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