### 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
}
			} else if (print_fps || GLOBAL_GET("debug/settings/stdout/print_fps")) {
				print_line(vformat("Project FPS: %d (%s mspf)", frames, rtos(1000.0 / frames).pad_decimals(2)));
			}
		} else {
			hide_print_fps_attempts--;
		}
		Engine::get_singleton()->_fps = frames;
		performance->set_process_time(USEC_TO_SEC(process_max));
		performance->set_physics_process_time(USEC_TO_SEC(physics_process_max));
		performance->set_navigation_process_time(USEC_TO_SEC(navigation_process_max));
		process_max = 0;
		physics_process_max = 0;
		navigation_process_max = 0;
		frame %= 1000000;
		frames = 0;
	}
	iterating--;
	/
	if (Input::get_singleton()->is_using_input_buffering() && !agile_input_event_flushing) {
		Input::get_singleton()->flush_buffered_events();
	}
	if (movie_writer) {
		movie_writer->add_frame();
	}
	if ((quit_after > 0) && (Engine::get_singleton()->_process_frames >= quit_after)) {
		exit = true;
	}
	if (fixed_fps != -1) {
		return exit;