Fix bilinear stencil tangent computions
Fix bilinear stencil tangent computions

- fix a couple of sneaky typos in the FarstencilTablesFactory

fixes #319

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