CAD Objects: Delete before Close-Polygon cross-pollutes areas

[we7u]

If draw one polygon, closing it, then draw another, but don't close, then delete that second one and start drawing a third: The third will attach to the first. Deleting should probably force a close.

[we7u]

Areas to look at:
src/objects.c:Draw_CAD_Objects_erase_selected()
src/objects.c:Draw_CAD_Objects_erase()
src/objects.c:Draw_CAD_Objects_close_polygon()

[we7u]

Deleting should force a close of the previous polygon, as well as trying to start a new polygon.