specctra_export.cpp: fix a bug in my export function to 3D viewer which generates bad shapes for holes generated by outlines
inside the main board outlines.
specctra_export.cpp: always approximate arcs in outlines by 36 segm for 360 degrees,
even for arc angle which differs from 90 degrees.
90 degrees is the default when creating an arc, but the arc value can be edited and have any value between 0 and 360 degrees.
Previously, 9 segments were used regardless the arc angle.