Pocket.py seems broken

[linuxras]

I have tried using pocket.p several times and failed and I think I've finally figured out why that is.
Lets say you have a scenario where you want a .5in deep pocket and you set zstep to .125in and zsafe to .25in with other relevant sizes. The cut seems to just do air cuts everytime.
The problem turns out to be that the first move is a G0 Z this would take you to .25 above the surface the next move is a G0 to XY center of the part, the problem comes at the next step with is a G1 G91 Z -, in this case this takes the tool to .125 above the surface and starts air cutting.
That can be fixed by changing the from G91 to a G90 move.

Another issue is dragging the tool at cut height back to center shudder I'll try to fix this up and add it to my pull request