### 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
docs.add_doc(class_doc);
			}
		}
		if (doc_tool_path == ".") {
			doc_tool_path = "./docs";
		}
		Ref<DirAccess> da = DirAccess::create_for_path(doc_tool_path);
		err = da->make_dir_recursive(doc_tool_path);
		ERR_FAIL_COND_V_MSG(err != OK, false, "Error: Can't create GDScript docs directory: " + doc_tool_path + ": " + itos(err));
		HashMap<String, String> doc_data_classes;
		err = docs.save_classes(doc_tool_path, doc_data_classes, false);
		ERR_FAIL_COND_V_MSG(err != OK, false, "Error saving GDScript docs:" + itos(err));
		OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
		return false;
	}
#endif /
	if (!doc_tool_path.is_empty()) {
		/
		Engine::get_singleton()->set_editor_hint(true);
		/
		if (!locale.is_empty() && locale != "en") {
			load_doc_translations(locale);
		}
		{
			Ref<DirAccess> da = DirAccess::open(doc_tool_path);
			ERR_FAIL_COND_V_MSG(da.is_null(), false, "Argument supplied to --doctool must be a valid directory path.");
		}
#ifndef MODULE_MONO_ENABLED
		/
		/