Synchronize the Z-Through Setting with Regular Path Generation

[BarbourSmith]

When cutting out parts on a CNC router it can be useful to generate a tool path which cuts slightly deeper than the part to ensure that the part is cut all the way through. This can help account for the wood thickness varying slightly or the z-axis zero point not being set exactly on the top surface of the wood.

The Z Thru setting takes this into account and allows the user to cut a little bit extra through the part, however the z-thru behavior could be improved.

Currently the way that z-thru works is that it adds additional passes after the regular passes have been calculated.

This can be inefficient because it always creates an extra pass even when it is not needed.

For example in the case shown below we have a 20mm thick part being cut out with a step down of 100mm and a z-thru setting of 2mm.

Ideally we would like to cut this part out in a single 22mm deep pass. Instead we are getting one 20mm pass and then a second 2mm deep pass despite the fact that our step down setting of 100 would more than allow for a single 22mm deep pass. This results in the cut taking twice as long.