### 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
if (Engine::get_singleton()->is_editor_hint() && p_check_run_in_editor) {
		
		return false;
	} else {
		if (XRServer::get_xr_mode() == XRServer::XRMODE_DEFAULT) {
			return GLOBAL_GET("xr/openxr/enabled");
		} else {
			return XRServer::get_xr_mode() == XRServer::XRMODE_ON;
		}
	}
}
String OpenXRAPI::get_default_action_map_resource_name() {
	String name = GLOBAL_GET("xr/openxr/default_action_map");
	return name;
}
String OpenXRAPI::get_error_string(XrResult result) {
	if (XR_SUCCEEDED(result)) {
		return String("Succeeded");
	}
	if (instance == XR_NULL_HANDLE) {
		Array args;
		args.push_back(Variant(result));
		return String("Error code {0}").format(args);
	}
	char resultString[XR_MAX_RESULT_STRING_SIZE];
	xrResultToString(instance, result, resultString);
	return String(resultString);
}
String OpenXRAPI::get_swapchain_format_name(int64_t p_swapchain_format) const {