* `flat` provides an easy to use API for 2D games based on sprites.
Clients of this API must redefine `App::update` to update the game logic and fill `App::sprites` with objects to draw.
+* `depth` defines an API for 3D games.
+ It is based on a list of `actors`, with `Model` composed of `Mesh` and `Material`.
+
* `cameras` provides cameras classes to produce MVP matrices which can be fed to shaders.
* `limit_fps` redefines existing services of gamnit to limit the framerate to a client-defined value.