Adds a Support Gap option - fixes case when support extrusion width is very small

when the support extrusion width is very small, then support ends up printed too close to object and is very hard to remove.

Although this is not the ideal fix for the support problems, this is important when the support extrusion width is small to prevent support from being too close.

note - the default is set to 150% of support extrusion width, so it should match your original math.

Another note, which is unfortunate, I don't know how to scale the value by the 1,000,000 that the $flow->scaled_width was scaled to... so it's hard-coded.

anyway, hope this is helpful.