implement PangoLayout::get_extents()

[d47081]

https://docs.gtk.org/Pango/method.Layout.get_extents.html

<?php

// initialize
Gtk::init();

// init some testing data
$layout = new PangoLayout(
    (new GtkDrawingArea)->create_pango_context()
);

$layout->set_text("test", 4);

var_dump(
    $layout->get_extents() // OK
);