title: Draw border around nontransparent part of image on canvas
body: <p>I'm drawing an image onto a canvas using <code>drawImage</code>. It's a PNG that is surrounded by transparent pixels, like this:</p>
  
  <p><img src="https://i.stack.imgur.com/YLF4F.png" alt="isometric drawing of a patch of grass"></p>
  
  <p>How can I add a solid-colored border to the visible part of that image on the canvas? To clarify: I don't want a rectangle that surrounds the image's bounding box. The border should go around the grass patch.</p>
  
  <p>I did consider using shadows, but I don't really want a glowing border, I want a solid one.</p>