Fix accumulation of stencil derivatives
Fix accumulation of stencil derivatives

this time without a typo

fixes #364

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