Add basic deinterlacing support, i.e. bob-deinterlacing whereby only
the selected field from the input surface is kept for the target surface.
Setting gst_vaapi_filter_set_deinterlacing() method argument to
GST_VAAPI_DEINTERLACE_METHOD_NONE means to disable deinterlacing.

Also move GstVaapiDeinterlaceMethod definition from vaapipostproc plug-in
to libgstvaapi core library.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>