Allow actors drawing a shape (instead of an image)

Nobody has claimed this yet.

When teaching the first steps with Pygame zero, kids tend to get lost in:

- Searching the right images for their game.
- Where did they download it? Where / what is the "images" directory?.
- Move the new images to the right directory.
- Make sure that the images are correctly named.
- Resizing the images!

Would it be possible to allow the actors to also use shapes like rectangles and squares?

I'm thinking of something like

```py
paddle = Actor()
paddle.shape = Rect((x, y), (w, h))
paddle.color = r, g, b
# paddle.fill_color = r, g, b

def draw()
paddle.draw_rect()
# paddle.draw_filled_rect()
```

Start by tracing the Actor drawing entry point and the proposed Rect-based API shown in the issue. Determine how an Actor without an image should represent its shape, color, position, and dimensions, then define completion around rectangles or squares rendering correctly through the proposed draw methods.

Written by the indexing model from the issue text.