libhb: add new function for retrieving previews

libhb: add new function for retrieving previews

This new function has a couple advantages over the old one (which we
should phase out).

It does not require hb_job_t as a parameter, instead it uses hb_ui_geometry_t
which is a smaller and simpler struct. The entire job struct is overkill as
input to this function.

It returns an hb_image_t that fully describes the returned image instead of
just a uint8_t array. The caller does not have to make assumptions about image
size, line stide, or pixel format since hb_image_t specifies these things.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6242 b64f7644-9d1e-0410-96f1-a4d463321fa5