The algorithm for generating the current slot index is a simple round robin,
nonetheless it's not assured that the next slot index it's not still used by a
still living encode picture.
This new way holds an array with the still living encode pictures and the next
slot index looks for a released index in the array.
Its downside is deallocating a picture need to be removed from the array, so the
helper has to be passed to the uninit() function
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8007>
8fc2af44c8