### 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
}
	});
#ifdef TOOLS_ENABLED
	if (editor) {
		packed_data->set_disabled(true);
		main_args.push_back("--editor");
		if (!init_windowed) {
			init_maximized = true;
			window_mode = DisplayServer::WINDOW_MODE_MAXIMIZED;
		}
	}
	if (!project_manager && !editor) {
		
		project_manager = !found_project && !cmdline_tool;
	}
	if (project_manager) {
		Engine::get_singleton()->set_project_manager_hint(true);
	}
#endif
	GLOBAL_DEF("debug/file_logging/enable_file_logging", false);
	
	
	
	
	GLOBAL_DEF("debug/file_logging/enable_file_logging.pc", true);
	GLOBAL_DEF("debug/file_logging/log_path", "user://logs/godot.log");
	GLOBAL_DEF(PropertyInfo(Variant::INT, "debug/file_logging/max_log_files", PROPERTY_HINT_RANGE, "0,20,1,or_greater"), 5);
	if (!project_manager && !editor && FileAccess::get_create_func(FileAccess::ACCESS_USERDATA) &&
			GLOBAL_GET("debug/file_logging/enable_file_logging")) {