Move Renderable into its own file.
Make the guts of Renderable private.
Change Renderable to take centered positions.
Convert Renderable to World Coordinates internally.
Convert AnimationWithOffset to world coords.

Animations (via Actor.CenterPosition) now
understand Altitude, so there is potential for
mis-positioned animations if any existing altitude
hacks were missed.
Convert Parachute to world coords.
Convert ChronoshiftPower to world coords.
Convert building rendering to world coordinates.
Rewrite RenderCargo -> WithCargo.

Now uses world coordinates and properly displays
cargo at all facings.
Turn Renderable into an abstract class and implement SpriteRenderable.
Round PxPosition to the nearest pixel.
Remove obsolete functions from WorldRenderer.