Fix z-ordering

[brainsmoke]

Screen coordinates are used in the calculation of baryonic coordinates, but the calculation of the z-coordinate is done in a way that breaks the invariant of straight lines remaining straight.

This fix divides the z-coordinate by the same as the x and y coordinates are divided.

z' = (z-1)/z

This projects z = [1..+inf] onto z' = [0..1]

Reproducible problem:

scale([.1,50,10])
rotate([90,0,90])
cylinder(1,1,1,$fn=3);
translate([-4.01,0,0])
sphere(4);

[brainsmoke]

[osresearch]

Before:

With this patch:

I think this makes things worse? The before is definitely not perfect with some of the edges disappearing.

[osresearch]

Re-reverted in https://github.com/osresearch/plotter-vision/commit/5d775804893526816a479e70082aebb3007e97f3