title: How do I get text bounding box drawn with pgmagick
body: <p>I successfully draw some text on an image by pgmagick:</p>
  
  <pre><code>from pgmagick.api import Image
  img = Image((300, 200))
  img.annotate('Hello World')
  img.write('helloworld.png')
  </code></pre>
  
  <p>but how to get the bounding box of text? Anyone can help me?</p>