### 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
String base_path = GLOBAL_GET("debug/file_logging/log_path");
		int max_files = GLOBAL_GET("debug/file_logging/max_log_files");
		OS::get_singleton()->add_logger(memnew(RotatedFileLogger(base_path, max_files)));
	}
	if (main_args.size() == 0 && String(GLOBAL_GET("application/run/main_scene")) == "") {
#ifdef TOOLS_ENABLED
		if (!editor && !project_manager) {
#endif
			const String error_msg = "Error: Can't run project: no main scene defined in the project.\n";
			OS::get_singleton()->print("%s", error_msg.utf8().get_data());
			OS::get_singleton()->alert(error_msg);
			goto error;
#ifdef TOOLS_ENABLED
		}
#endif
	}
	if (editor || project_manager) {
		Engine::get_singleton()->set_editor_hint(true);
		use_custom_res = false;
		input_map->load_default(); 
	} else {
		input_map->load_from_project_settings(); 
	}
	if (bool(GLOBAL_GET("application/run/disable_stdout"))) {
		quiet_stdout = true;
	}
	if (bool(GLOBAL_GET("application/run/disable_stderr"))) {
		CoreGlobals::print_error_enabled = false;
	};