Add helper header to obtain native anchor pointer from xr anchor (#195)

* update babylonnative

* change branch reference for now

* add native anchor access

* fix helper header to compile

* update babylonnative

* move back to correct repo reference

* Update Modules/@babylonjs/react-native-windows/windows/include/XrAnchorHelperOpenXR.h

Co-authored-by: Ryan Tremblay <[email protected]>

* expose ability to get native anchor

Co-authored-by: Ryan Tremblay <[email protected]>

Author: C. M. Barth

Modules/@babylonjs/react-native-windows/windows/include/XrAnchorHelperOpenXR.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#if __has_include("jsi/jsi.h")
+#include "jsi/jsi.h"
+#include <openxr/openxr.h>
+
+namespace BabylonReactNative
+{
+    bool TryGetNativeAnchor(facebook::jsi::Runtime& jsiRuntime, facebook::jsi::Value& jsAnchor, XrSpatialAnchorMSFT& nativeAnchor)
+    {
+        nativeAnchor = nullptr;
+        if (!jsAnchor.isObject())
+        {
+            return false;
+        }
+
+        if (!jsiRuntime.global().hasProperty(jsiRuntime, "navigator"))
+        {
+            return false;
+        }
+
+        auto navigator{ jsiRuntime.global().getProperty(jsiRuntime, "navigator").asObject(jsiRuntime) };
+        if (!navigator.hasProperty(jsiRuntime, "xr"))
+        {
+            return false;
+        }
+
+        auto nativeXr{ navigator.getProperty(jsiRuntime, "xr").asObject(jsiRuntime) };
+        if (!nativeXr.hasProperty(jsiRuntime, "getNativeAnchor"))
+        {
+            return false;
+        }
+
+        auto getNativeAnchor{nativeXr.getPropertyAsFunction(jsiRuntime, "getNativeAnchor")};
+        auto nativeAnchorPtr{static_cast<uintptr_t>(getNativeAnchor.call(jsiRuntime, { jsAnchor.asObject(jsiRuntime) }).asNumber())};
+
+        nativeAnchor = reinterpret_cast<XrSpatialAnchorMSFT>(nativeAnchorPtr);
+        return true;
+    }
+}
+#endif