[ptx] automatch: another attempt at semi-automatic panorama stitching

[Bruno Postle]

I've played a bit more with my automatch script for finding
stitching control points.

-  I tried the optimisation Pablo suggested, it got slower.  I guess
   I'm not very good at this sort of thing :-p.

-  I figured that by just using the first 40 feature-points (instead
   of all 100 points provided by klt) gives a speedup of about 15x.
   It now runs in about 20 seconds instead of 10 minutes, though it
   only detects 40% as many points.

-  It is incredibly sensitive to accurate lens correction, I tried
   to match uncorrected photos and got no results at all (note that
   it can be modified to take account of lens parameters, but they
   will still need to be known beforehand).

The process for determining which control-points to use is purely
statistical, but the thresholds I am using are entirely arbitrary.
These decisions about which points to discard would benefit greatly
from a proper statistical understanding.

Slightly updated version that uses fewer feature-points and writes
better .pto files:

   http://bugbear.blackfish.org.uk/~bruno/distance/automatch.pl

--
Bruno