lens: apply or correct lens distortions

this implements a very simple distortion correction following The Double Sphere Camera Model Vladyslav Usenko, Nikolaus Demmel and Daniel Cremers.

parameters

center move the center of the projection
scale scale the image larger or smaller
squish0 the radius of one of the spheres
squish1 the other radius used for the mapping
ca some fake chromatic aberration correction, just scales differently for the rgb channels