vp9alphadecodebin: Fix auto-pluging v4l2slvp9dec

vp9alphadecodebin: Fix auto-pluging v4l2slvp9dec

This adds the alignment field to the template caps. Without this field
set, the auto-plugger will see fixed caps and will use
gst_caps_is_subset() against the caps produced by the parser. This is a
challenge for all cases where a parser can do conversion. This is fixed
by adding alignment field, which makes the auto-pluggers do an
intersection of the caps as it gets unfixed caps after intersection now.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1439>