func ClearDepthf(d float32) {
	syscall.Syscall(gpClearDepthf, 1, uintptr(math.Float32bits(d)), 0, 0)
}
func ClearDepthf(d float32) {
	syscall.Syscall(gpClearDepthf, 1, uintptr(math.Float32bits(d)), 0, 0)
}
// specify the clear value for the depth buffer