### 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 (tablet_driver.is_empty()) {
			tablet_driver = DisplayServer::get_singleton()->tablet_get_driver_name(0);
		}
	}
	for (int i = 0; i < DisplayServer::get_singleton()->tablet_get_driver_count(); i++) {
		if (tablet_driver == DisplayServer::get_singleton()->tablet_get_driver_name(i)) {
			DisplayServer::get_singleton()->tablet_set_current_driver(DisplayServer::get_singleton()->tablet_get_driver_name(i));
			break;
		}
	}
	if (DisplayServer::get_singleton()->tablet_get_current_driver().is_empty()) {
		DisplayServer::get_singleton()->tablet_set_current_driver(DisplayServer::get_singleton()->tablet_get_driver_name(0));
	}
	print_verbose("Using \"" + tablet_driver + "\" pen tablet driver...");
	
	rendering_server = memnew(RenderingServerDefault(OS::get_singleton()->get_render_thread_mode() == OS::RENDER_SEPARATE_THREAD));
	rendering_server->init();
	
	rendering_server->set_render_loop_enabled(!disable_render_loop);
	if (profile_gpu || (!editor && bool(GLOBAL_GET("debug/settings/stdout/print_gpu_profile")))) {
		rendering_server->set_print_gpu_profile(true);
	}
	if (Engine::get_singleton()->get_write_movie_path() != String()) {
		movie_writer = MovieWriter::find_writer_for_file(Engine::get_singleton()->get_write_movie_path());
		if (movie_writer == nullptr) {
			ERR_PRINT("Can't find movie writer for file type, aborting: " + Engine::get_singleton()->get_write_movie_path());
			Engine::get_singleton()->set_write_movie_path(String());
		}
	}
#ifdef UNIX_ENABLED