depth-first bug omitting lower layers

[stewartoallen]

using this workspace, the lower parts of the channel are missing when depth first is enabled. happens with roughing and pocket operations.

pocket-alternative-to-roughing.kmz.zip

[stewartoallen]

polygon offsetting is producing "jaggies" in the lower areas of the trench and then those are not nesting under the polygons above thus they're eliminated from the descent