Remove automatic validity check at end of render

In more complex cases, the final Nef_Polyhedron is_valid check() took up to 30 % of the total rendering time just to be able to say Valid: YES. In the case of cached geometry, the validity check was totally dominating the execution time when doing a render. This patch removes the automatic validity check, instead adding a menu command "Check Validity".

This is just a suggestion – please merge or discard. :-)