pose-extract gradientAscent(): distance interpolation, dynamic walkdir + step limit

2x2 interpolation allows us to estimate the distance more smoothly,
avoiding aliasing effects while determining the control points.

We now do not keep walkdir constant, but update it in each step as
if we started in that step. This leads to better guidance to the
center of the worm, but may also lead to oscillations, so we introduce
a limit on the maximum number of steps of the algorithm.
pose-extract backbone progress figure: Show on top of edgedists heightmap inst. of blob
pose-extract processFrame(): Fill any holes after thresholding