code: func StencilFuncSeparate(face uint32, xfunc uint32, ref int32, mask uint32) {
  	syscall.Syscall6(gpStencilFuncSeparate, 4, uintptr(face), uintptr(xfunc), uintptr(ref), uintptr(mask), 0, 0)
  }
docs: // set front and/or back function and reference value for stencil testing