GL_SGIX_shadow
http://oss.sgi.com/projects/ogl-sample/registry/SGIX/shadow.txt
GL_TEXTURE_COMPARE_SGIX 0x819A
GL_TEXTURE_COMPARE_OPERATOR_SGIX 0x819B
GL_TEXTURE_LEQUAL_R_SGIX 0x819C
GL_TEXTURE_GEQUAL_R_SGIX 0x819D