### 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
StringName ibt = script_res->get_instance_base_type();
							bool valid_type = ClassDB::is_parent_class(ibt, "Node");
							ERR_CONTINUE_MSG(!valid_type, vformat("Script does not inherit from Node: %s.", info.path));
							Object *obj = ClassDB::instantiate(ibt);
							ERR_CONTINUE_MSG(!obj, vformat("Cannot instance script for autoload, expected 'Node' inheritance, got: %s."));
							n = Object::cast_to<Node>(obj);
							n->set_script(script_res);
						}
					}
					ERR_CONTINUE_MSG(!n, vformat("Path in autoload not a node or script: %s.", info.path));
					n->set_name(info.name);
					/
					to_add.push_back(n);
					if (info.is_singleton) {
						for (int i = 0; i < ScriptServer::get_language_count(); i++) {
							ScriptServer::get_language(i)->add_global_constant(info.name, n);
						}
					}
				}
				for (Node *E : to_add) {
					sml->get_root()->add_child(E);
				}
				OS::get_singleton()->benchmark_end_measure("load_autoloads");
			}
		}
#ifdef TOOLS_ENABLED
		EditorNode *editor_node = nullptr;
		if (editor) {
			OS::get_singleton()->benchmark_begin_measure("editor");
			editor_node = memnew(EditorNode);