### 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
const double time_scale = Engine::get_singleton()->get_time_scale();
	MainFrameTime advance = main_timer_sync.advance(physics_step, physics_ticks_per_second);
	double process_step = advance.process_step;
	double scaled_step = process_step * time_scale;
	Engine::get_singleton()->_process_step = process_step;
	Engine::get_singleton()->_physics_interpolation_fraction = advance.interpolation_fraction;
	uint64_t physics_process_ticks = 0;
	uint64_t process_ticks = 0;
	uint64_t navigation_process_ticks = 0;
	frame += ticks_elapsed;
	last_ticks = ticks;
	const int max_physics_steps = Engine::get_singleton()->get_max_physics_steps_per_frame();
	if (fixed_fps == -1 && advance.physics_steps > max_physics_steps) {
		process_step -= (advance.physics_steps - max_physics_steps) * physics_step;
		advance.physics_steps = max_physics_steps;
	}
	bool exit = false;
	/
	XRServer::get_singleton()->_process();
	for (int iters = 0; iters < advance.physics_steps; ++iters) {
		if (Input::get_singleton()->is_using_input_buffering() && agile_input_event_flushing) {
			Input::get_singleton()->flush_buffered_events();
		}
		Engine::get_singleton()->_in_physics = true;
		uint64_t physics_begin = OS::get_singleton()->get_ticks_usec();
		PhysicsServer3D::get_singleton()->sync();
		PhysicsServer3D::get_singleton()->flush_queries();
		PhysicsServer2D::get_singleton()->sync();
		PhysicsServer2D::get_singleton()->flush_queries();
		if (OS::get_singleton()->get_main_loop()->physics_process(physics_step * time_scale)) {