title: pygame: mask non-image type surfaces
body: <p>I know how to mask images. But if I simply want to create a circle and mask it, it would be annoying to create an image of a simple circle outside of my code, especially when I need to create different kinds of circles. Appearantly you can create Rect objects in pygame but there is no class Circle.
  <code>pygame.mask.from_surface</code> requires a <code>surface</code>. Can I pass a non-image type surface as a parameter? If so how can I mask circles and/or other objects?</p>
  <p>Here's something I imagined which obviously throws an error:</p>
  <pre><code>circle = pygame.Circle((10, 10), 5) # (center coordinates), radius
  pygame.mask.from_surface(circle)
  </code></pre>