Only count positive E delta towards deposition

Some slicers retract while performing moves that do not contribute to
deposition.
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Add austerus-verge --zmin mode

This mode returns the bounds of the region in which the Z axis is less
than zmin. Once we have determined a safe working Z height from existing
depositions on plate we can find the region in which a new piece of gcode
violates this (so we can check this region is clear).
First hash at shift
Always output
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