n-fcyls2 / n-fcyls Primitives

[Philippe Hurbain]

More thoughts on this...
   
The proposed new primitives allow to complete a cylinder segment cut by a sloped plane going through a pair of vertices. While this may seem restrictive, this situation often occurs, eg. when a stud meets a cut plane.

• As far as I can tell, only prims covering up to 1/4 cylinder are needed.
  
• But it could make sense for cyls2 to cover up to 1/2 cylinder to simplify usage and limit number of vertices    
  
• 1-4fcyls2 = 1-4cyls2 and 1-4fcyls = 1-4cyls