autorectify_cv(): Turn off per-tile debugging displays and some debug prints

This way, we allow for more massive sampling.
autorectify_cv(): n_samples 6 -> 16
measure_rectification_one(): Disable debug print
autorectify_cv(): Factor out sample_rp_from_tiling()
autorectify_cv() fine-tuning: Fine-tune just lens position in the loop, not actual rp
autorectify_cv(): Factor lens0 access to RectifyParams.lens0()
autorectify_cv(): Factor out finetune_lens_position()
RectifyParams.median(): Minimize offset distance from center
Autorectify finetune_lens_position(): Raise IndexError when going out of picture
Autorectify finetune_lens_position(): Do not go to north-west when all choices are equal (typically 1. when in pitch black area)
autorectify_cv(): Refine RectifyParams based on refining lens further away along the axes

At this point, we *finally* seem to reach robust auto-rectification
behavior even for just very slightly tilted images. Yaaaay!
autorectify_cv(): Disable showing window with tile rectangles