mxfdemux: Store GstMXFDemuxEssenceTrack in their own fixed allocation

mxfdemux: Store GstMXFDemuxEssenceTrack in their own fixed allocation

Previously they were stored inline inside a GArray, but as references to
the tracks were stored in various other places although the array could
still be updated (and reallocated!), this could lead to dangling
references in various places.

Instead now store them in a GPtrArray in their own allocation so each
track's memory position stays fixed.

Fixes ZDI-CAN-22299

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3055

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5638>