fix bug with zero width tree supports throwing exception.
This a PR from @scottmudge commited to OrcaSlicer.
More than the original commit, we also unify the usage of line_width and
support_extrusion_width, making things simpler.
Change-Id: I0b7a4344cbda4fdf05dfd9f8965f598ad6844429
(cherry picked from commit 4a7c4d4a0cc4507e7eadfb1825b1fd11bf0aba52)