use pixel thresholds relative to image size

[johnwlambert]

Make pixel thresholds a function of the image size, to avoid treating 480p and 1296p images differently.

A good reference is http://phototour.cs.washington.edu/ModelingTheWorld_ijcv07.pdf: We set the RANSAC outlier threshold to be 0.6% of the maximum image dimension, i.e., 0.006 max(image width,image height) (about six pixels for a 1024×768 image).

COLMAP users also complain about how in COLMAP this is hard-coded in their issue #1278.