#360 SDS artifacts with creases

[Chris Foster]

Artifacts appear when rendering SDS with creases. The following forum thread lays out the issue in more detail:

http://community.aqsis.org/2009/03/more-sds-detail-issues.html

This may be due to the recent changes which push SDS to the limit surface; the limit surface calculation ignores the presence of creases. Ignoring creases at subdivision levels greater than some threshold is perfectly consistent, but artifacts may appear between patches with different maximum subdivision level.