Consider GPX track segments when interpolating

[peterneubauer]

The GPX scripts is right now flattening the points from all tracks into one big list. That means that - if you stopped the GPX logging and started it again, you will have several tracks in the GPX fiel with gaps in between. Images in these gaps should be cosidered outside the GPX coverage (like before/after the whole file) and not be modified. Right now the images get falsely interpolated directions where there is no GPX coverage.