gstcevidenc: Add option to set interlace encoding.

When interlace enconding is enabled, input buffers are treated
as interleave fields, even lines the top field and bottom lines
the bottom field. The codec will be configured to do interlace
encoding per field. So for each incoming buffer the encoder
sends two output buffers (one per field).
gstceh264enc: Add a property to enable interlace encoding and a property
to define the interlace reference mode.