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 gst-plugins-base#825 (closed)