Add pixel perfect collision detection

Hi, many thanks for creating Pgzero, it's working really well at our Coder Dojo in Brighton :)

I've hacked together some code for pixel perfect collision detection, would you consider adding it to pgzero? I can submit a pull request if you are happy to review.

Here's the code:

```
def collide_pixel(actor1, actor2):

# Get masks for pixel perfect collision detection:
for a in [actor1, actor2]:
if not hasattr(a, 'mask'):
a.mask = mask.from_surface(a._orig_surf)

# Offsets based on current positions of actors
xoffset = int(actor2.left - actor1.left)
yoffset = int(actor2.top - actor1.top)

# Check for overlap = collision
return actor1.mask.overlap(actor2.mask, (xoffset, yoffset))
```

Bit more background here:
http://chalkpath.co.uk/blog/html/2020/01/12/pixel_perfect_collision_detection_with_pygamezero.html

Start with the proposed collide_pixel function in the issue and read the linked pixel-perfect collision article for context. The issue names no repository files or tests, so first locate the existing actor and collision APIs, then define where this capability belongs and how it should be tested. Done means the feature is integrated consistently with Pgzero and its behavior is covered.

Written by the indexing model from the issue text.