### 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 (script_res->can_instantiate()) {
			StringName instance_type = script_res->get_instance_base_type();
			Object *obj = ClassDB::instantiate(instance_type);
			MainLoop *script_loop = Object::cast_to<MainLoop>(obj);
			if (!script_loop) {
				if (obj) {
					memdelete(obj);
				}
				OS::get_singleton()->alert(vformat("Can't load the script \"%s\" as it doesn't inherit from SceneTree or MainLoop.", script));
				ERR_FAIL_V_MSG(false, vformat("Can't load the script \"%s\" as it doesn't inherit from SceneTree or MainLoop.", script));
			}
			script_loop->set_script(script_res);
			main_loop = script_loop;
		} else {
			return false;
		}
	} else { /
		if (!editor && !ClassDB::class_exists(main_loop_type) && ScriptServer::is_global_class(main_loop_type)) {
			String script_path = ScriptServer::get_global_class_path(main_loop_type);
			Ref<Script> script_res = ResourceLoader::load(script_path);
			if (script_res.is_null()) {
				OS::get_singleton()->alert("Error: Could not load MainLoop script type: " + main_loop_type);
				ERR_FAIL_V_MSG(false, vformat("Could not load global class %s.", main_loop_type));
			}
			StringName script_base = script_res->get_instance_base_type();
			Object *obj = ClassDB::instantiate(script_base);
			MainLoop *script_loop = Object::cast_to<MainLoop>(obj);
			if (!script_loop) {
				if (obj) {
					memdelete(obj);