### 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
/
		GLOBAL_DEF("dotnet/project/assembly_name", "");
		GLOBAL_DEF("dotnet/project/solution_directory", "");
		GLOBAL_DEF(PropertyInfo(Variant::INT, "dotnet/project/assembly_reload_attempts", PROPERTY_HINT_RANGE, "1,16,1,or_greater"), 3);
#endif
		Error err;
		DocTools doc;
		doc.generate(doc_base);
		DocTools docsrc;
		HashMap<String, String> doc_data_classes;
		HashSet<String> checked_paths;
		print_line("Loading docs...");
		for (int i = 0; i < _doc_data_class_path_count; i++) {
			/
			String path = _doc_data_class_paths[i].path;
			if (path.is_relative_path()) {
				path = doc_tool_path.path_join(path);
			}
			String name = _doc_data_class_paths[i].name;
			doc_data_classes[name] = path;
			if (!checked_paths.has(path)) {
				checked_paths.insert(path);
				/
				Ref<DirAccess> da = DirAccess::create_for_path(path);
				err = da->make_dir_recursive(path);
				ERR_FAIL_COND_V_MSG(err != OK, false, "Error: Can't create directory: " + path + ": " + itos(err));
				print_line("Loading docs from: " + path);
				err = docsrc.load_classes(path);
				ERR_FAIL_COND_V_MSG(err != OK, false, "Error loading docs from: " + path + ": " + itos(err));
			}