Watertight solids for gCAD3D

gCAD3D does not provide watertight solids yet. All curves are polygonalized individually; the common boundary of two sufaces side-by-side is polygonalized twice; this can cause small slits between the boundaries. gCAD3D V2.35 changes: - all curve-modifications (eg CUT) create now trimmed-curves eg the result of trimming a circle is a (trimmed -) curve; eg "S20 = CUT C20 L20" // trimmed-curve S20 = cut circle C20 with line L20 - all curves now have a polygonal-representation with BoundaryMarks the display- function uses the polygonal-representation; the functions for polygonal-representation are PRCV_*. TODO V2.36: - modify surface-functions to use the polygonal-representation of its boundaries