Like UYVY and similar formats this is rounding down to the start of the previous macro-pixel to not mix up the different components. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1988>
It's not handling any kind of complex packed format, only formats that are like YVYU. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1988>