libhb: automatically add rendersub filter when required

libhb: automatically add rendersub filter when required

This requires the addition of a filter->post_init function to inform
filters of the final job configuration after all filters have been
initialized.  Rendersub needs to know cropping, but cropping isn't known
till after crop_scale filter is initialized. Since crop_scale is initialized
*after* rendersub is initialized, post_init is needed.  Currently, rendersub
is the only filter that defines post_init.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6830 b64f7644-9d1e-0410-96f1-a4d463321fa5