### Godot version

4.1

### System information

Windows 10 - Godot 4.1.1 - Compatibility

### Issue description

When trying to use the Varjo XR3 headset (in OpenXR mode) with Godot 4 (with OpenXR and passthrough activated), the passthrough does not work, i.e. the scene is rendered on a black background instead of the camera stream.

It seems to me that are two problems in the openxr module, more precisely in the openx_api.cpp file (fixing them solves  the issue) : 

1. The XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT  is only set when the number of layers is > 1, which is fine for the Meta Quest headset but does not work for the Varjo since it only requires one layer. 
2. The environment blend mode XR_ENVIRONMENT_BLEND_MODE_ALPHA_BLEND  is only really set when the XRFbPassthrough extension is present, which is not the case with the Varjo runtime. 

I am not sure how to fix this without causing issues for other headsets.

1. Would it be an issue to always activate the XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT ? 
2. Could blend modes be activated even if they are not supported by an extension ? 





### Steps to reproduce

- Create a Godot 4 project with OpenXR activated in Compatibility mode
- Add the code to activate OpenXr and to enable passthrough : https://docs.godotengine.org/en/stable/tutorials/xr/openxr_passthrough.html#passthrough-extension
- Start the Varjo Base software and activate OpenXR
- Run the Godot project

### Minimal reproduction project

N/A
String hash = String(VERSION_HASH);
	if (!hash.is_empty()) {
		hash = "." + hash.left(9);
	}
	return String(VERSION_FULL_BUILD) + hash;
}
#if defined(TOOLS_ENABLED) && defined(MODULE_GDSCRIPT_ENABLED)
static Vector<String> get_files_with_extension(const String &p_root, const String &p_extension) {
	Vector<String> paths;
	Ref<DirAccess> dir = DirAccess::open(p_root);
	if (dir.is_valid()) {
		dir->list_dir_begin();
		String fn = dir->get_next();
		while (!fn.is_empty()) {
			if (!dir->current_is_hidden() && fn != "." && fn != "..") {
				if (dir->current_is_dir()) {
					paths.append_array(get_files_with_extension(p_root.path_join(fn), p_extension));
				} else if (fn.get_extension() == p_extension) {
					paths.append(p_root.path_join(fn));
				}
			}
			fn = dir->get_next();
		}
		dir->list_dir_end();
	}
	return paths;
}
#endif
void initialize_physics() {