We never implemented correctly the selection of memories in a buffer given the
plane.
This patch uses gst_buffer_find_memory() for that. The offset is checked via the
video meta either in the input and output buffers, or the default offset given
the format and size.
This patch also requests the video meta option for the output buffers.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9219>