### 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
RenderingServer::get_singleton()->sync();
	/
	rendering_server->global_shader_parameters_clear();
	if (xr_server) {
		/
		/
		xr_server->set_primary_interface(Ref<XRInterface>());
	}
#ifdef TOOLS_ENABLED
	GDExtensionManager::get_singleton()->deinitialize_extensions(GDExtension::INITIALIZATION_LEVEL_EDITOR);
	uninitialize_modules(MODULE_INITIALIZATION_LEVEL_EDITOR);
	unregister_editor_types();
#endif
	ImageLoader::cleanup();
	GDExtensionManager::get_singleton()->deinitialize_extensions(GDExtension::INITIALIZATION_LEVEL_SCENE);
	uninitialize_modules(MODULE_INITIALIZATION_LEVEL_SCENE);
	unregister_platform_apis();
	unregister_driver_types();
	unregister_scene_types();
	finalize_theme_db();
	/
	finalize_navigation_server();
	finalize_physics();
	GDExtensionManager::get_singleton()->deinitialize_extensions(GDExtension::INITIALIZATION_LEVEL_SERVERS);
	uninitialize_modules(MODULE_INITIALIZATION_LEVEL_SERVERS);
	unregister_server_types();
	EngineDebugger::deinitialize();
	if (xr_server) {
		memdelete(xr_server);
	}