### 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
/
	List<String> cmdline_args = OS::get_singleton()->get_cmdline_args();
	BindingsGenerator::handle_cmdline_args(cmdline_args);
#endif
	if (use_debug_profiler && EngineDebugger::is_active()) {
		/
		/
		EngineDebugger::get_singleton()->profiler_enable("scripts", true);
	}
	if (!project_manager) {
		/
		/
		/
		/
		rendering_server->global_shader_parameters_load_settings(!editor);
	}
	_start_success = true;
	ClassDB::set_current_api(ClassDB::API_NONE); /
	print_verbose("CORE API HASH: " + uitos(ClassDB::get_api_hash(ClassDB::API_CORE)));
	print_verbose("EDITOR API HASH: " + uitos(ClassDB::get_api_hash(ClassDB::API_EDITOR)));
	MAIN_PRINT("Main: Done");
	OS::get_singleton()->benchmark_end_measure("scene");
	return OK;
}
String Main::get_rendering_driver_name() {
	return rendering_driver;
}
/
static MainTimerSync main_timer_sync;
bool Main::start() {