### 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
Ref<DirAccess> da = DirAccess::open(local_game_path.substr(0, sep));
							if (da.is_valid()) {
								local_game_path = da->get_current_dir().path_join(
										local_game_path.substr(sep + 1, local_game_path.length()));
							}
						}
					}
				}
			}
			local_game_path = ProjectSettings::get_singleton()->localize_path(local_game_path);
#ifdef TOOLS_ENABLED
			if (editor) {
				if (game_path != String(GLOBAL_GET("application/run/main_scene")) || !editor_node->has_scenes_in_session()) {
					Error serr = editor_node->load_scene(local_game_path);
					if (serr != OK) {
						ERR_PRINT("Failed to load scene");
					}
				}
				DisplayServer::get_singleton()->set_context(DisplayServer::CONTEXT_EDITOR);
				if (!debug_server_uri.is_empty()) {
					EditorDebuggerNode::get_singleton()->start(debug_server_uri);
					EditorDebuggerNode::get_singleton()->set_keep_open(true);
				}
			}
#endif
			if (!editor) {
				DisplayServer::get_singleton()->set_context(DisplayServer::CONTEXT_ENGINE);
			}
		}
		if (!project_manager && !editor) { /