idzio: Prepare for switchable backends

idzio/xi.h

@@ -0,0 +1,9 @@
+#pragma once
+
+/* Can't call this xinput.h or it will conflict with <xinput.h> */
+
+#include <windows.h>
+
+#include "idzio/backend.h"
+
+HRESULT idz_xi_init(const struct idz_io_backend **backend);