Remove some cast warnings that are triggered in gcc 4.7 (seen in Fedora 18)
Remove some cast warnings that are triggered in gcc 4.7 (seen in Fedora 18)

fixes #223

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