videodecoder: don't copy interlace-mode from reference state
User is passing the actual interlace-mode when calling gst_video_decoder_set_interlaced_output_state() so it should not be overidden by the input interlace-mode.
Needed to fix #825 (closed) as we want to keep interlace-mode=interleaved from parsers and have the OMX decoder producing interlace-mode=alternate.
Related MRs:
Edited by Guillaume Desmottes