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