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 gstreamer/gst-plugins-base#825

Part-of: <gstreamer/gst-omx!73>
10 jobs for backport-mr-72-into-1.18-fix-alternate in 18 minutes and 38 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #4948533

00:00:13

passed android docker #4948532

00:02:17

passed cerbero fedora amd64 docker #4948531

00:02:12

passed fedora amd64 docker #4948530

00:03:19

passed gst-indent amd64 docker #4948534

00:00:11

 
  Preparation
passed gst indent #4948536

00:00:12

passed manifest #4948535

00:03:35

 
  Build
passed build gst-omx tizonia fedora x86_64 #4948538

00:05:49

passed build gst-omx zynq fedora x86_64 #4948537

00:02:36

 
  Integrate
passed documentation #4948539

00:11:42