Add 9-patch to examples

[damusss]

After a short discussion with @Starbuck5 9-patch scaling is a recipe like process that doesn't involve algorithmic pixel manipulation so an example is probably good enough for it.

This is a good place to come every time you need your own 9-patch function and good to teach for GUI purposes. It's also existing in every other game engine so at least an example is needed.

I made the 9-patch code behave like a transform function would, adding comments and type hinting.

[damusss]

Attaching a preview of the example for ease of review

[bilhox]

Overall the example is really cool. I would apprecate since you mentionned it works for any custom surface to actually use a custom surface, like an image with the necessary properties to make it work.

Anyways, good job :)

[damusss]

With my las commit I

• used pygame.Vector2.elementwise
• changed the while True
• added a real image on top of the rect

[bilhox]

LGTM, I left some requested changes.

[MyreMylar]

OK, LGTM 👍

[bilhox]

LGTM!