Better support for creating your own surfaces

In [lordmauve/snowgame](https://github.com/lordmauve/snowgame) we encountered several places where we needed to bypass Pygame Zero's APIs in order to create new surfaces, blit actors to them, and set pixels.

In some ways it was always intended that you could directly access Pygame, but I was surprised at how little Pygame Zero functionality could be used once you do.

For example, it would be useful if Actors could take a surface that they were attached to as a parameter, and optionally have it overridden in Actor.draw().

It would be useful to wrap new "Screen" objects around newly created surfaces.

It would be useful to be able to create a "Screen buffer" by copying the dimensions of a screen.

Start by reviewing the existing Actor and Screen APIs and how they interact with Pygame surfaces. Compare the requested capabilities: attaching Actors to supplied surfaces, overriding the surface in Actor.draw(), wrapping new Screen objects, and copying screen dimensions for a buffer. Done means these surface workflows are defined and supported consistently.

Written by the indexing model from the issue text.