Blender Git Loki

Tracking: Cleanup, de-duplicate implementation of marker lookup

The logic was duplicated.

Should be no functional changes. The modified function is expected
to give same exact results for all inputs.

On the "caching last-used track" topic. The code was using last_marker
to allow faster lookup of marker closest to the frame. With this
change it is still the case since the BKE_tracking_marker_get() does
cache last used marker.