Fix maxvalence calculation in FarMeshFactory
Fix maxvalence calculation in FarMeshFactory

The lowest max valence should always be 4: even a triangle
generates 4 quads.

fixes #249

shengfuintel/OpenSubdiv,c64kernal/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,shengfuintel/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,xldeveloper/OpenSubdiv,jcowles/OpenSubdiv,anshuarya/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,nyotis/OpenSubdiv,xldeveloper/OpenSubdiv,anshuarya/OpenSubdiv,c64kernal/OpenSubdiv,takahito-tejima/OpenSubdiv,jcowles/OpenSubdiv,poljere/OpenSubdiv,takahito-tejima/OpenSubdiv,c64kernal/OpenSubdiv,poljere/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,lookup1980/OpenSubdiv,anshuarya/OpenSubdiv,nyotis/OpenSubdiv,takahito-tejima/OpenSubdiv,lookup1980/OpenSubdiv,shengfuintel/OpenSubdiv,poljere/OpenSubdiv,xldeveloper/OpenSubdiv,nyotis/OpenSubdiv,lookup1980/OpenSubdiv,jcowles/OpenSubdiv