Adds Viewport class.
Viewport is sent RenderManager. Only components inside the current
viewport will be drawn. Each component is drawn relative to the viewport
as well.
Don't let RenderComponents draw themselves.
RenderComponents return an sf::Drawable and a position that the
RenderManager then uses together with the viewport to draw at the final
position.
Rename "Managers" struct to SceneData
Use viewport in SceneManager.
When adding a Scene also, also add a corresponding Viewport.