osd: fix conversion warnings in glDrawContext
osd: fix conversion warnings in glDrawContext

warning C4242: 'argument' : conversion from 'int' to 'unsigned char', possible loss of data

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