### 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 (err == OK && !assign.is_empty()) {
							if (project_manager) {
								if (assign == "interface/editor/project_manager_screen") {
									init_screen = value;
									break;
								}
							} else if (editor) {
								if (assign == "interface/editor/editor_screen") {
									init_screen = value;
									break;
								}
							}
						}
					}
				}
			}
		}
		if (found_project && EditorPaths::get_singleton()->is_self_contained()) {
			if (ProjectSettings::get_singleton()->get_resource_path() == OS::get_singleton()->get_executable_path().get_base_dir()) {
				ERR_PRINT("You are trying to run a self-contained editor at the same location as a project. This is not allowed, since editor files will mix with project files.");
				OS::get_singleton()->set_exit_code(EXIT_FAILURE);
				return FAILED;
			}
		}
	}
#endif
	
	input = memnew(Input);
	
	{