Feature proposal: world built-in

pgzero is currently very suited to making games that take up only a single screen.

I propose the addition of a _world_ built-in to allow easy management and realization of games with content outside of a single screens space.

As a first idea, I think _world_ could be an object of a class _World_ which has two main features. A set of position offsets for X and Y and a list of Actors. The X and Y offsets would allow the "camera position" to move by changing the absolute X and Y positions of everything in the world. The X and Y positions of each individual actor in the world would instead refer to their position in the world. This is realized by having the world be drawn with _world.draw()_. This function would go through each actors, change their position by the world offset, call _.draw()_ on them and then change their position back.

This would obviously be quite unperformant so a change to the _.draw()_ function with an optional parameter or a modified new function to draw at a specific position could allow more optimized drawing for lots of actors in the world.

I'm happy to get any feedback or ideas on this. I think something along these lines could really expand what's easily possible with pgzero. A natural expansion might see the world being rotatable.

Start by reviewing the existing Actor drawing behavior and the proposed World/world.draw() interface described in the issue. Define how X and Y camera offsets, world-positioned Actors, and drawing at specific positions should work; the feature is done when games can manage and render content beyond a single screen.

Written by the indexing model from the issue text.