image-to-gcode improvements

With advice primarily from #emc's <lernaen_hydra> I have started to add some more advanced features to image-to-gcode. These latest changes are in the EMC2 CVS tree.

First, the milling order has new choices: In addition to the naive "Positive", "Negative" and "Alternating" you can now choose "Up Milling" and "Down Milling" which will split the row or column depending on the shape of the path.

Second, "perpendicular lace with with reduced toolpaths": the second scan mills only steep areas, and the first scan can avoid areas that are steep in the direction of the second scan.

torus.ngc was generated with several of these new options enabled.

The next things to add are probably roughing passes and more intelligent entry and exit moves (including a separate feed for the entry move).

(originally posted on the AXIS blog)