Blender Git Loki

Subsurf: Introduce quality option

For users it defines how accurate vertex positions are in terms
of limit surface (as in, how close the vertices locations to the
condition when they are calculated for an infinitely subdivided
mesh).

This affects things like:

- Irregular vertices (joint of 3 or more edges)
- Crease

Keep quality value low for performance.

NOTE: Going higher does not necessarily mean real improvement
in quality, ideal case might be reached well before maximum
quality of 10. Quality of 3 is a good starting point.

Internally quality is translated directly to adaptive subdivision
level.

Reviewers: brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D3599