func ClearDepth(depth float64) {
	syscall.Syscall(gpClearDepth, 1, uintptr(math.Float64bits(depth)), 0, 0)
}
func ClearDepth(depth float64) {
	syscall.Syscall(gpClearDepth, 1, uintptr(math.Float64bits(depth)), 0, 0)
}
// specify the clear value for the depth buffer