release vertexbuffer on exit viewer
release vertexbuffer on exit viewer

Closes #28

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