mxfdemux: Refactor pull seek

mxfdemux: Refactor pull seek

In order to figure out the exact start position (backed by a keyframe) accross
all tracks, we first figure out the backing keyframe position, and *then* seek
to that position.

Avoids ending up in situations where we would properly seek to the backing
keyframe on video ... but not on the audio streams (they would have been set to
the original non-keyframe position). Fixes key-unit seeking.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2371>