static PFNGLFRAMEBUFFERRENDERBUFFEREXTPROC vglFramebufferRenderbufferEXT = NULL;
static PFNGLDELETERENDERBUFFERSEXTPROC vglDeleteRenderbuffersEXT = NULL;
static PFNGLFRAMEBUFFERRENDERBUFFEREXTPROC vglFramebufferRenderbufferEXT = NULL;
static PFNGLDELETERENDERBUFFERSEXTPROC vglDeleteRenderbuffersEXT = NULL;