### 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
}
	OS::get_singleton()->add_frame_delay(DisplayServer::get_singleton()->window_can_draw());
#ifdef TOOLS_ENABLED
	if (auto_build_solutions) {
		auto_build_solutions = false;
		/
		if (!editor) {
			OS::get_singleton()->set_exit_code(EXIT_FAILURE);
			ERR_FAIL_V_MSG(true,
					"Command line option --build-solutions was passed, but no project is being edited. Aborting.");
		}
		if (!EditorNode::get_singleton()->call_build()) {
			OS::get_singleton()->set_exit_code(EXIT_FAILURE);
			ERR_FAIL_V_MSG(true,
					"Command line option --build-solutions was passed, but the build callback failed. Aborting.");
		}
	}
#endif
	return exit;
}
void Main::force_redraw() {
	force_redraw_requested = true;
}
/* Engine deinitialization
 *
 * Responsible for freeing all the memory allocated by previous setup steps,
 * so that the engine closes cleanly without leaking memory or crashing.
 * The order matters as some of those steps are linked with each other.
 */
void Main::cleanup(bool p_force) {