pose-extract extendToTip(): Clean up walkDir initialization
pose-extract: Fix backbone spanning multiple blobs

Edges crossing dark areas are removed from the complete graph. This works
fine (for now) as long as one of the blobs is small as it's unlikely that
point #0 will be in it. Also, this is unfortunately very slow; working on
getting numba to work with this.
pose-extract-lf pointsToBackbone(): Eliminate points that have identical coordinates
pose-extract-lf pointsToBackbone(): Fix point elimination so that edges to later removed points are not created
pose-extract gradientAscent(): Better protection against point walking out of frame
pose-extract lineSumValue(): Do not leave the picture when walking in direction