### 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
extension_ptrs.ptr() 
	};
	copy_string_to_char_buffer(project_name, instance_create_info.applicationInfo.applicationName, XR_MAX_APPLICATION_NAME_SIZE);
	XrResult result = xrCreateInstance(&instance_create_info, &instance);
	ERR_FAIL_COND_V_MSG(XR_FAILED(result), false, "Failed to create XR instance.");
	
	XrInstanceProperties instanceProps = {
		XR_TYPE_INSTANCE_PROPERTIES, 
		nullptr, 
		0, 
		"" 
	};
	OPENXR_API_INIT_XR_FUNC_V(xrGetInstanceProperties);
	result = xrGetInstanceProperties(instance, &instanceProps);
	if (XR_FAILED(result)) {
		
		print_line("OpenXR: Failed to get XR instance properties [", get_error_string(result), "]");
		runtime_name = "";
		runtime_version = "";
	} else {
		runtime_name = instanceProps.runtimeName;
		runtime_version = OpenXRUtil::make_xr_version_string(instanceProps.runtimeVersion);
		print_line("OpenXR: Running on OpenXR runtime: ", runtime_name, " ", runtime_version);
	}
	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
		wrapper->on_instance_created(instance);
	}
	return true;
}
bool OpenXRAPI::get_system_info() {