Camera world matrix should calculate from start of valid bundle

[stuarta0]

trafficstars

Code in export_bundler.py line 82 sets frame to scene.frame_start before calculating the camera matrix_world. In testing, it needs to be the frame where the movieclip tracking starts without error.

The following records the earliest any marker appears, but it's not the point where the tracking becomes valid.

frame_min = 99999
for t in clip.tracking.tracks:
    frame_min = min(frame_min, t.markers.values()[0].frame)

[stuarta0]

Workaround is to manually set the scene frame_start to the correct frame