Needlessly generates offset anim tracks

[furroy]

From what I can tell if you aren't using borders, the offset x&y for every frame of every anim is 0. I put in a check so that if all offset keys are 0x0 for an anim, it calls animation.remove_track(tracks.offset.idx) at the end to delete the unneeded anim track.

[bfloch]

The offsets are necessary when the imported sprites are "packed" by using the "Trim Cels" option in Aseprite.