omxvideodec: support interlace-mode=interleaved input

omxvideodec: support interlace-mode=interleaved input

interlace-mode=alternate is a special case of interlace-mode=interleaved
where the fields are split using two different buffers.

The Zynq decoder always produces alternate content and we
used to assume that upstream will set interlace-mode=alternate in its
caps as well.
This is no longer the case as h265parse is now setting
alternate-mode=interleaved on alternate content to not break compat with
elements not supporting alternate.
As a result the decoder now accept both 'interleaved' and 'alternate' on
its input and ensures that its ouput has interlace-mode=alternate.

Needed to fix https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/825

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-omx/-/merge_requests/72>