Part: Offset2D: fix save-load instability
Part: Offset2D: fix save-load instability

Fixes #2699
It was found out that when direction of normal found by FindPlane
depends on if we are using result of offset directly, or after copying.

The fix (workaround) is to copy offset result inside  offset routine, so
that no unstable behavior makes its way to the user.

usakhelo/FreeCAD,bblacey/FreeCAD-MacOS-CI,Fat-Zer/FreeCAD_sf_master,usakhelo/FreeCAD,Fat-Zer/FreeCAD_sf_master,usakhelo/FreeCAD,bblacey/FreeCAD-MacOS-CI,wood-galaxy/FreeCAD,wood-galaxy/FreeCAD,wood-galaxy/FreeCAD,wood-galaxy/FreeCAD,wood-galaxy/FreeCAD,bblacey/FreeCAD-MacOS-CI,bblacey/FreeCAD-MacOS-CI,Fat-Zer/FreeCAD_sf_master,bblacey/FreeCAD-MacOS-CI,usakhelo/FreeCAD,bblacey/FreeCAD-MacOS-CI,Fat-Zer/FreeCAD_sf_master,bblacey/FreeCAD-MacOS-CI,usakhelo/FreeCAD,usakhelo/FreeCAD,Fat-Zer/FreeCAD_sf_master,wood-galaxy/FreeCAD,usakhelo/FreeCAD