Blender Git Loki

Fix T79453: Motion Tracking: marker does not remember 'enabled' state

Caused by rB63ee3db96107.

While above commit corrected the clip offset, it also removed logic to
ensure a marker on a particular frame. This is needed though, otherwise
changes on a particular frame are applied to the marker being returned
by 'BKE_tracking_marker_get' which can be a completely different marker
if none exist for that frame yet.

This patch partly reverts rB63ee3db96107 and reintroduces the framenr
for the MarkerUpdateCb and uses that to ensure a marker on that frame.

Candidate for backporting to 2.83 LTS?

Reviewers: sergey, jacqueslucke

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