layerling 1.56.1

• Loose holes are cut on export: A hole left ungrouped was dropped from an STL, 3MF, OBJ or SVG export, though the view shades the cut and the STEP export already made it, so a part had to be grouped before every export. Now every visible hole that goes along is taken out of the bodies it reaches, as grouping would cut it; with a selection, only the selected holes count. The material estimate, the section SVG and the MCP bridge's layerling_export_section_svg count them the same way. Reported by @mobiusfix in #203.