### 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 (feature_number >= max_features) {
				max_features = feature_number;
				text_driver_idx = i;
			}
		}
	}
	if (text_driver_idx >= 0) {
		Ref<TextServer> ts = TextServerManager::get_singleton()->get_interface(text_driver_idx);
		TextServerManager::get_singleton()->set_primary_interface(ts);
		if (ts->has_feature(TextServer::FEATURE_USE_SUPPORT_DATA)) {
			ts->load_support_data("res://" + ts->get_support_data_filename());
		}
	} else {
		ERR_FAIL_V_MSG(ERR_CANT_CREATE, "TextServer: Unable to create TextServer interface.");
	}
	OS::get_singleton()->benchmark_end_measure("servers");
	MAIN_PRINT("Main: Load Scene Types");
	OS::get_singleton()->benchmark_begin_measure("scene");
	
	
	initialize_theme_db();
	register_scene_types();
	register_driver_types();
	register_scene_singletons();
	initialize_modules(MODULE_INITIALIZATION_LEVEL_SCENE);
	GDExtensionManager::get_singleton()->initialize_extensions(GDExtension::INITIALIZATION_LEVEL_SCENE);
#ifdef TOOLS_ENABLED
	ClassDB::set_current_api(ClassDB::API_EDITOR);
	register_editor_types();